How to do Slime

Homemade slime is very easy to do and give us hours of fun.

What we need:

• white glue,
• 2 plastic cups, trash it when you finish it,
• food coloring (green is the classic but you can make red or blue slime, chose your favorite color),
• water,
• borax (you can find it in drugstore),  
• plastic tablespoon,
• regular tablespoon.

How to:

1. Fill 1 cup with water;
2. Add a Borax tablespoon, use the plastic spoon and mix well,
3. Reserve this solution,
4. On the other cup measure about 1in of white glue,
5. Add 20ml of water (that's about 3 tablespoons),
6. Mix,
7. Add 4 drops of food coloring,
8. Add to the second cup 1 tablespoon of Borax solution,
9. Mix,
10. Wait 30seg,
11. Play with it.

NOTE THIS:

• Slime is totally safe for the skin.
• Keep the slime closed in a jar when you are not playing with it.
• Don't use it near your hair or carpets and rugs.
• Keep it away from the younger children and pets.
• Don't eat it.

Results:

The polymer formed is both a solid and a liquid. This polymer can behave like a liquid and take the shape of the glass, and yet, you can hold it in your hand like a solid. Substances like this one are called non-Newtonian fluids... and what is this? Exactly what we wrote before, or in scientific speech is a fluid whose viscosity is variable depending on the applied pressure.

You can make this demonstration an experience
Answer with your restless to the following questions:

• How does the Slime change with the Borax quantity in the first solution?
• How does the Slime change with the Glue quantity in the second solution?  
• What is the best way to store your slime? Try several yes of boxes, jars, bottles..
• Does the Borax trademark make a difference? Witch one works best?
• What about the glue?
• What is the ideal water quantity? Try to do several slimes with different water quantities.

NOTE THIS:
Never change 2 or more variables at the same time. For instance, if you change the water quantity keep the glue trademark the same.

Et Voilá!
You want fun? Play with non-Newtonian fluids

Enjoy!

Bending bones with vinegar, the rubber bone

We can have lots of fun with vinegar. We leave you with another demonstration with this stinky friend.
Bending bones
What? Bending bones? How?

Yes, bending bones, the effect works best if we use a bird bone, that’s because this bones are hallow.

What we need:

• a jar, with lid, big enough to the bone,
• 1 chicken bone, Lucky bone or the leg bone works best,
• Vinegar, no flavors, no additions.

How to:

1. Wash the bone, with running water;
2. Show the bone to your restless, show him/her how hard is to bend it;
3. Place the bone on the jar;
4. Cover it with vinegar;
5. Screw the bottle cap;
6. Let it rest for 3 days, place the jar in a safe place, you don’t want your kitchen smelling like vinegar;
7. Remove the bone;
8. Wash it on running water;

Results:
Bend it, tcham tcham! Is it really a rubber bone? Well it looks like one

Why?
Like our bones, chicken bone have calcium, this mineral gives it hardness.
What does the vinegar do to make this happen?
Vinegar is a weak acid, but it's strong enough to dissolve the bone calcium. Once the calcium gone there is nothing more to make it hard- all that remains is the soft bone tissue. Just like what happened here with the naked egg.
 
Ask your restless to answer this questions:

• Does the number of hours the bone is in the vinegar affects the final outcome? How?
• Does the size of the bone affect the demonstration? How?
• Using different kinds of vinegar will lead to different results?

Et voilá!
A rubber bone

Enjoy!

The air takes space- Try this

Another simple and basic science demonstration. Restless Minds already played with air a few times, check it out.

What we need:

• flask with lid, metallic works best,
• nail,
• hammer,
• straw,
• molding clay,
• water.

How to do it:

1. Use the nail, and with the hammer punch a hole on the lid big enough for the straw. Don't let your child use the hammer;
2. Fill the flask with water to 2/3, you can use juice;
3. Close the flask with the lid;
4. Place the straw in the hole;
5. Seal the hole with the clay;
6. Drink it, using the straw.

Results:
You can't drink. At beginning this looked a bit stupid right? But now you understand.

Why?
When we use the straw we don’t really think about what happens in the plastic tube, but the straw only works because the liquid we suck is replaced with air. In fact is all about pressure.

• When, in optimal conditions, we place a straw in a drink, the air pressure in the drink in exactly the same inside the liquid on the straw.
• When we pull the liquid up, we remove some air from inside the straw, and the pressure inside the plastic tube drops.
• While this is happening inside the straw the pressure on the surface of the liquid remains the same.
• This pressure variation, between surface and straw interior, makes the liquid to be pushed through the tube.
• If we close the flask, and prevent the air from circulating freely there is no way to create a pressure differential and therefore there isn't nothing to push the liquid into the straw!

Et voilá!
The juice will stay there, until you unseal the jar!

Enjoy!

Needle magic trick

We saw here several demonstrations that give incredible magic shows. It'e a fact the balloon trick is an oldie, but is also true that sometimes the simple is the most interesting.

Before anything else: When does a balloon bursts? When the air inside is suddenly expelled, otherwise the balloon doesn't burst, just empties.
We say it bursts because it makes a big BOOM!
 
What we need:

• balloon,
• sewing pins ,
• strong tape,

How to:

1. Fill the balloon half way;
2. Glue small pieces of tape, 2, 3 ,4 ... as you wish, it will depends on the number of pins you have and on balloon size;
3. Make sure that the tape is well glued to the balloon, air bubbles will lead to a big BOOM!;
4. Gently, but without fear, stuck a pin in the center of duct tape;
5. Repeat this to all pieces of tape in the balloon.

What happens?
Balloon doesn't burst!

Why?
The balloon burst when the air is suddenly forced out the rubber coat. In this demonstration it doesn't burst because we made a "seal"- with the tape- around the spot where we pin it, here the rubber is reinforced.

We can try to take the pins of the balloon, which will make the balloon emptying slowly. While the tape
resist the balloon will not burst.

Try to observe the rubber ripping beneath the tape, near the spot where you place the pin, when the air pressure, inside the balloon, overcome the tape "seal" we can observe a small tear underneath the  tape.  The more the balloon is filled, the greater the pressure is, and faster the pressure overcomes adhesive and tear appears, the next step is Bang!

Note:

• Glue the tape pieces apart from each other;
• Use light color balloons, it will be easier to observe;
• Use strong tape, works best

Variations:
You can try to understand better this pin magic:

• Use several balloons, same sized; fill them with different air quantities. Use a chronometer and see how long the air pressure takes to beat the rubber "seal". 
• What is the ideal volume air?
• Use more then 1 pin, and on each balloon vary the space between the tape pieces. 
• Does this have any influence on the time the balloon takes to lose the air? 
• And does it simply lose all the air or does it burst?

Et voilá!
It's magic

Enjoy!

The shrinking bottle

We already did several demonstrations here to prove that the air takes up space.
Today the demonstration is another way to play with the air.

What we need:

• plastic bottle, with cap,
• hot water,
• funnel.

How to:

1. Wash the bottle;
2. Let it dry;
3. Heat up the water, don’t let it boil, its not necessary;
4. With the funnel pour the water in the bottle;
5. Wait for 10 sec;
6. Reject the water to the sink;
7. Quickly cap the bottle again, you have to be fast.

What happen?
The bottle shrinks.
In other words, the bottle walls collapse.

Remember this? What happens to the bottle is exactly the same thing.
Before further explanations is very important to understand two things:
First: air exerts pressure on the bottle, like in everything around us, including ourselves.
Second: any forces system tends to look for equilibrium.

So,
The hot water heats the air inside the bottle. Try to close the bottle immediately after pour the water inside, the bottle will swell, that’s because hot air can't escape to atmosphere. When you reject the water, the air inside the bottle rapidly cool. The cool air occupies less space then the hot air, and, theoretically there will be a lot of "empty space" inside the bottle, because of that the plastic walls collapse under the pressure of the air outside the bottle.

Et voilá!
Another fun way to spend some time

Enjoy!

How to do a parachute toy

This is a old toy, no batteries needed, no updates or upgrades, just the plastic soldier and the plastic bag. Today we are going to see how to build a parachute and how it works.

We will need:

• plastic square with a 40cm (about 15in) side. You can use a garbage plastic bag,
• Scissors,
• Ruler,
• Pen,
• 8 strings with 30cm (12in) length,
• A small toy, like a plastic soldier.

How to:

1. On each side of square measure and mark, with a pen, 4, 8 and 12in, like the image bellow;
2. Cut off the shaded parts;
3. With the scissors punch a hole on each corner of the plastic, like the picture, holes must be small, large holes will compromise the parachute;
4. Attach one string to each punch you made, with a knot;
5. Attach the other string end to the toy; the toy must have no more then 10cm (4in) height.

   

   Figure2

   
   

   
   



Figure1

6. Throw the parachute from a height place, like a balcony; remember the goal is to land the toy gently.

Results:
We hope your parachute landed gently giving an opportunity to your toy to survive.
When you throw the parachute the toy (the weight) pulls down the strings, forcing the plastic to open. Now the parachute contact surface with the air is bigger, the air resistance is also bigger and that’s why the parachute goes down slowly. The greater the air resistance the smoothly the parachute falls.

Try this with your restless:

• Throw the parachute from different heights, keep a record of your results and variables:
• Compare the falling times,
• The parachute always opens?
• Cut a small hole on center of the plastic, this will help the air to escape:
• Does it fall faster with or without the hole?

Et Voilá!
A plastic soldier parachute

Enjoy!

Giant soap bubbles

Blowing Bubbles

Dip your pipe and gently blow.
Watch the tiny bubble grow
Big and bigger, round and fat,
Rainbow-colored, and then
SPLAT!

Margaret Hillert
Yes, today we are going to play with giant soap bubbles.
Not much to say about them, just that they make children smile and adults dream ... give a child a bubble, and its like you are giving her/him the magic itself.

What we need:

• 2 cups of washing dish detergent,
• 3 cups of warm water,
• four ounces of glycerine, buy it at the drugstore, the secret ingredient,
• large bucket, preferably with a lid,
• "Hula Hoop",
• small stool,
• rubber pool, the hula hoop must fit

How to:

1. Mix all ingredients in the bucket, water, detergent and glycerine;
2. Upon mixing the solution pour it in the pool;
3. Put the bow in the pool;
4. Place the stool inside it;
5. Equip your restless with goggles, fins, swimming suit, and whatever else you can remember of;
6. Lift the Hula Hoop in order to cover the child, he will found himself inside a giant soap bubble!;
7. Now, let your restless try it, replace him/her in the pool.

For better results:
The weather must be warm, wet weather works best
Heavy water makes this demonstration more difficult

But what happen in a soap bubble?

Surface tension
H2O molecules are in red and white
Blue circle:
hydrophilic end of the soap molecule,
Black "tail":
hydrophobic end of sap molecule

The soap is the main ingredient on a soap bubble; moreover, if you want to try, brands and different types of dishwasher detergent give different results. Soap is made of molecules with two distinct ends, one hydrophilic and another one hydrophobic. The first one attracts the water, the second repels it.
If we could cut a bubble we would see something like the picture, a water layer surrounded by two soap layers, one inside the other outside the bubble.
The interaction between the soap bubbles pushes the water molecules away from each other relieving surface tension.
On the other hand glycerine bonds with hydrogen in water preventing evaporation.
 
But a question remains:
With the hula hoop we can make spherical giant soap bubbles, but if we use a square form can we make square bubbles?...
 
Answer
No
 
Why?
Because everything in Nature happens with the minimum possible energy spent. This is a basic fact of Nature.
Surface tension of the bubble "is going to shape" it in a form with the less possible area to contain that volume. That shape is the sphere- also called bubble- the most effective shape in universe when comes to energy saving. Look the Sun, the planets, the water drops, the berries, and I am sure you can remember a few more spheres

Et Voilá!
Who would say we can learn with sop bubbles?
 
Enjoy!

Osmosis demonstration using eggs

Remember this? Today we are going t need 2 of those, maybe more if you want to go a little further.
Today we are going to play and learn about osmosis.

Before continuing:
Cells have several transport mechanisms, in other words, several different ways to make the molecules to pass from inside to outside and vice versa. On of those mechanisms is called osmosis, and we can say for sure "that's all about concentration".

Osmosis is the movement of solvent molecules through a membrane from a region of  lower solute  concentration, to a region of higher concentration.
In other words, and considering the solvent is water, osmosis is the movement of water molecules from "in" or "out" the cell, aiming to match the concentration on both sides of the membrane.

In general we only can see a cell with a microscope. You don’t need a super microscope like electronic, a optical is enough.

In Nature there is a group of cells witch don't need a microscope to be "seen”, the eggs of species that "lay eggs." Ostrich eggs are the biggest eggs (not extinct specie), humming birds are the smallest, to "make" one ostrich egg we need 24 chicken eggs.

The egg is a cell, full of potential, to become a new being; only our frying pan or a starving predator can stop it from happen.
This cell have, like any other cell, a membrane (works as envelope for white and yolk), this membrane is double layered and is easy to observe on the blunt region where it forms a air pouch.

Let's start our demonstration. Before anything else we need to "undress" 2 eggs, you can see how here.

Ready?
Let’s go

What we need

• 2 jars with lid, big enough to place the egg inside,
• marker, to write on glass,
• water,
• 2 naked eggs,
• honey.

How to:

1. Put a "naked egg" in each jar;
2. Fill the first with water;
3. Fill the second with honey;
4. Close the two jars;
5. Label them with the marker, write on the first "H2O" or "water" and in the second "honey";
6. Place the jars in the refrigerator;
7. Wait 24 hours;
8. Take the eggs of the jars.
9. What do you see?

Observations:
The egg in the water jar looks like swollen and smooth.
The egg in the honey jar looks like wrinkled.

Why?
The white part of the egg has 90% water and the honey has only about 18%. We placed our cell (the egg) in two different environments, the water (with 100% water molecules, or close enough) and the honey (with about 18% H2O).

In the first case the water flow goes from a 100% environment to a 90% environment, from the water to the cell (egg), theoretically until the two concentrations are set at 95%, the egg swells.
In the second jar the water flows from the 90% environment to the 18% environment, from the cell to the water, theoretically until the two concentrations are set at 54%, the egg wrinkles.

At this process, where solvent molecules move (in this example H2O) to find a equilibrium concentration, in two different environments, separated by a membrane, we call osmosis.

Image:

• |X| here the outside H2O concentration match the inside concentration;
• |>X| here the concentration outside the cell/egg membrane is lower then the concentration inside it, the honey example.
• |<X| here the concentration outside the cell/egg membrane is higher then the concentration inside it, the water flask example;

Make this an experiment
Try other variables; remember to write down all your observations:

• Use water with food coloring;
• Use alcohol;
• Use salt water;
• Use Vinegar;
• Or simply, place your "honey naked egg" in a jar with water and wait another 24h, what do you think it will happen?

Sources:
http://www.victorialodging.com/recreation/birding/small-big-fast-slow
http://fisiologia.med.up.pt/Textos_Apoio/Membranas/Membranas.pdf
http://www.benefits-of-honey.com/honey-nutrition.html
Et voilá!
Play with eggs, learn from them

Enjoy!

Black hole magic

Today we are going to do some magic!
Your restless will be amazed!

What we need:

• chose a easy cleaning surface, as always, to avoid problems,
• rubber gloves,
• metal can, any size will do
• Styrofoam, like the ones in the images.

How to:

1. Put your gloves on;
2. Make a large pile of Styrofoam;
3. Pour the acetone in the can, about 1 in high;
4. Ask your restless to set aside the quantity of Styrofoam he/she thinks to fill the can;
5. Drop the pieces into the can one by one;
6. Keep dropping;
7. The pile is finish? Grab some Styrofoam from the big pile and keep dropping;
8. If everything goes as planed the big pile will disappear before your eyes into the can, like it was a "black hole" that grabs everything around it.

Results:
Styrofoam vanish in the can!

The actors:
The acetone is an organic compound (CH3)2CO, is a highly flammable liquid with a high level of solvency. A lot of compounds dissolve on acetone, like nail polish.
Styrofoam is made of polystyrene. Manufactures use a blown agent that helps to transform the polystyrene in Styrofoam, the product this procedure originate is a highly porous and rigid polymer, with high percentage of air, trapped on porous

What happened?
In fact what happen is very easy to explain. Styrofoam is a polymer, made of several chemical units linked (like all polymers), acetone works on this links breaking them. Once the Styrofoam is made by thousands of air bubbles, all that remains at the end of the chemical reaction is: air.
Styrofoam is a very light material and with a lot of air inside, that makes it perfect o disappear with a small quantity of acetone.

After all where did the Styrofoam go?
When Styrofoam dissolves in acetone it doesn't disappear. Don’t forget  the statement "In nature, nothing is created, nothing is destroyed, everything is transformed". Polystyrene molecules continue in solution. Styrofoam solubility is very important when comes to recycling. Polystyrene in acetone solution can be extracted and used to make new materials avoiding its accumulation in sanitary landfills.

Caution:
Kids must be kept away from acetone,
Use other acetone then the nail polish remover for better results.
Verify if what you are using is really Styrofoam and isn't other similar biodegradable product

Et voilá!
It's a kind of magic!
Tcham tcham!

Enjoy!

Explosive lunch bag

Explosions are most of the time messy. Usually it's dangerous, but what Restless Minds bring you today is a nice, cool and harmless prank. 

This demonstration uses the same reaction we saw here, with the volcano, and here, with the balloon.
In this reaction acetic acid reacts with sodium bicarbonate, the product of that reaction is carbon dioxide and water. 

What we need:

• zip lock plastic bag,
• paper napkins,
• 2 tablespoons of baking soda, sodium bicarbonate,
• 1/2 cup of vinegar, without any flavors,
• 1/4 cup of warm water.

How to:

1. First of all, do not do this at home, do it outside in a place you can clean easily.
2. Make sure bag has no holes, fill it with water the easiest way,
3. Empty the bag,
4. Dry it,
5. Place the baking soda on the napkin,
6. Fold it like an envelope,
7. Inside the zip lock bag place the vinegar and the warm water;
8. Quickly but carefully  place the "envelope" inside;
9. Close the zip;
10. Place the bag on the floor and step back, again quickly but carefully;

What happen?
The bag will swell and open with a BANG!

Results:
As we saw here and here, acetic acid (vinegar) reacts with sodium bicarbonate (baking soda) and neutralizes it.
This reaction is very fast and results in the production of CO2- a gas. This gas fill the balloon raising the pressure inside, when the pressure is unsustainable the bag open with a loud BANG!


Chemical reaction:

from here

Et Voilá!

Enjoy!

Spinning coin inside a ballon- centripetal and centrifugal forces

Today we are going to talk about forces. Yes, those from physics. For the most of young restless minds this is a lot of formulas that make no sense at all.

In fact physics studies natural world, formulas are just the way we have to express that knowledge. Everything we see in a formula we can see in nature, the opposite is not true... yet

The demonstration we show here today only uses a coin and a balloon.
Easy and simple, like most of the great things in nature. This demonstration is perfect to show you what are centripetal and centrifugal forces.

What we need:

• small coin, 1cent or 5 cents;
• balloon;

How to:

1. Place the coin inside the balloon;
2. Inflate the balloon;
3. Make a knot;
4. Move the balloon, from bottom to top or upside down, rotate the balloon in your hand. Do this until the coin gain a steady circular motion.

What happened?

First the coin goes up and down, hits the balloon wall with an uncoordinated movement, after a while the coin begins a  steady circular motion, the coin will keep motion if we keep rotate the hand.

Why?
In a circular movement, centripetal and centrifugal forces are the same value, but with opposite signals, that means each of them pushes the coin in opposite directions. Centripetal force attracts the coin to the center of rotation, on the other hand centrifugal force pushes the same coin far from this center.
The coin maintains its movement because these two forces are in equilibrium with a third one, the tangential force, which makes the coin move inside the balloon.

Some examples of this dynamic are:
A carousel, on which the bars where the vehicles are placed exert a centripetal force, while the person in the vehicle will feel a centrifugal force (1).
When describing a curve with a car, the friction of the road exerts a centripetal force on the wheels so it does not get out of the curve (1).

Another simple observation we can do with this balloon is Newton First Law of Movement, like we did with the egg

How to:

• Spin the balloon until the coin start the steady circular motion;
• Stop moving your hand, quickly place the balloon on the table and let it go.

What happened?
The balloon will keep spinning, like the egg did.

Why?
Because the coin in its interior will keep moving for a few more seconds, and like the Newton First Law of Movement says, if something is moving or stationary, it will stay that way unless something disturbs the system.

References: 
(1)força centrípeta. In Infopédia. Porto: Porto Editora, 2003-2011. Disponível em http://www.infopedia.pt/$forca-centripeta;

Et Voilá!
A coin, a balloon, a demonstration and lots of fun!

Enjoy!

Fire proof ballons

Balloons are fun, in fact they are the funniest thing in the world for a young child. You can blow them up, you can lose it, you can make them fly with He, they can be bend to form animals... or they can be part of a magical moment.
Today we will show you another funny thing you can do with balloons.  It's very simple but requires the presence of an adult, from beginning to end of the test

We will need:

• 2 balloons,
• 1 candle,
• water, cold, or a piece of ice,
• funnel,
• Matches or a lighter.

How to:

1. With the help of the funnel put a little water in one of the two balloons;
2. Fill the two balloons, so that they are the same size;
3. Place the candle inside the sink, or another place you can wet without much trouble;
4. Light the candle, the adult should do this;
5. Touch the flame with the balloon that doesn’t have water. What happens?
6. Now try the other one, flip the balloon around slowly. What happens?

The first balloon burst almost immediately, the second balloon, which contain water, takes longer to break (depending on the amount of water you put in there it may not even burst, it also depends on the rubber quality).

Why?

Both the first balloon and the second (with water) heat.
On the first one the flame heats the balloon, the rubber loses the properties that allow it to withstand the pressure of air that is inside, and consequently bursts.
The second balloon, the presence of water changes everything. In fact when the balloon is placed in the flame its not the balloon that gets hotter but the system water / balloon. The flame heats the rubber in contact with water, and the heat is transferred to the liquid, thereby allowing the rubber of the balloon to cool, and maintain properties longer.

Water absorbs heat very well. We need a huge amount of heat to change the water temperature. Notice the time it takes to heat water in a kettle and compare it with the time it takes to heat the kettle itself (do not touch the kettle!)
On the other hand, the cooling process of water releases a tremendous amount of heat, this contributes to the fact that the areas near oceans - or other bodies of water - are not so cold in winter as areas further inland.

These is a demonstration, turn this in an experiment by varying the amount of water in the balloon, or the water / air system, the time it takes to blow the balloon is influenced by these factors?
Record your results in a notebook, then try to make a graph that reflects the results, there is a direct relationship?

I leave you this video,

Et voilá!
The heat transfer at our fingertips

Enjoy!

Invisible Ink, easy to write hard to read

Secret messages, they love it! To be able to write something on a piece of paper that no one can read is amazing.

Today we are going to see 3 ways to write something that no one can read but you, you will doubt about which the paper sheet was used.

Invisible ink with baking soda
We already used baking soda on several demonstrations and fun games, today we will use it to write…

We will need:

• 1 cup,
• 60ml of water,
• The same amount of sodium bicarbonate,
• toothpicks,
• paper. 

How to:

1. Measure 60ml of water;
2. Pour water into the cup;
3. Measure 60 ml of sodium bicarbonate, this is solid, so it may seem strange measure in ml, but do it :);
4. Mix baking soda in water;
5. Mix it until the solution is homogenized;
6. Get your paper, and write your message with the toothpick.

How do you read it? 
Spread grape juice over the sheet; use a sponge or a brush. 
NOTE: the grape juice leaves stain.

Why? 
Sodium bicarbonate is a chemical compound that has a basic pH, the grape juice is acid, and the reaction between the two rise to a third colour, visible to the naked eye.

Invisible ink with milk

We need:

• Milk,
• Toothpick,
• Paper sheet.

How to:

1. Put some milk in a bowl;
2. With the toothpick or brush to write the secret message;
3. Let dry completely.

How do you read it? 
There are several hypotheses; all of them require a heat source, and the supervision of an adult.
You can iron the sheet, place a cloth between the iron and the message so the iron dot get dirt, you can heat the sheet next to a lamp, be careful so it doesn’t get on fire, or, and finally, you can use the stove.

Why? 
Milk is an organic product, and burns at a slower speed than the paper. When you heat the paper, the milk is leaves a brownish colour and you can read the message in the white sheet.

Invisible ink with lemon:

We need:

• Lemon juice,
• Toothpick,
• Paper sheet.

How to:

1. Place a little lemon juice on a plate;
2. With the toothpick or brush write the secret message;
3. Let dry completely.

How do you read it? 
As in the the milk ink, you can pass the iron sheet, the sheet can heat with a lamp, or you can use the stove, either of these alternatives requires adult supervision. There is another way to see the message by putting salt on the sheet, in this case wait about 60sec and clean the salt, then, and carefully pass a crayon over it.

Why? 
For the same reason that points to the use of milk

In addition to these dyes, we can use other products such as white wine, vinegar, other fruit ...
You can make this demonstration in an experiment using other liquids that you think that might work.
Register which works best and what are the safest methods to read, do not forget that the aim is to get a top-secret message without being caught.

Et voila!
Nobody will notice that blank sheet on the table.

Enjoy!