What we need:
- straight pins,
- toilet paper,
- Fill the bowl with water;
- Wait a few seconds until the water stop moving;
- Can you place a straight pin floating in the water? Try it;
- What happened?
- Try a second pin.
- Place the pin on the top of a piece of toilet paper;
- Place both things in the water, very carefully and gently
- Wait a few seconds.
The paper sinks, the straight pint floats.
Surface tension, that's the answer.
When we place the pin in the water, without using the paper, it immediately sinks, its weight is too high for the area it occupies, ie is very dense.
The paper, by contrast have much area for the weight, in fact it doesn't sink, actually it soaks, in other words water molecules fill the paper pores in it's porous structure and fill the voids in the web cellulose paper. this way the paper becomes heavier and sinks.
Surface tension is responsible for what one might call "skin."
The water surface is formed by a barrier of water molecules. This barrier is what allows insects to land on water, the soap bubbles to exist, and the pin does not sink.
The first pin sinks because it doesn't start from a position of equilibrium and rest, unlike the second which is resting on the paper. The paper when sinks exerts sufficient force on the skin of the water to bend it but not to break it, and the pin floats.
It's magic! No, it's science!