The skin on Liquids

Take a glass and fill it with water. When it is full right up to the top, stand it on a kitchen table and try adding more water to it, a drop at a time. Do this carefully and without shaking the glass. You will find that you can keep adding the drops until the water is above the rim of the glass.

Look at the top of the water and you will see a kind of skin stretching across it, which stops the water spilling out. All liquids have a kind of skin, caused by surface tension.

Look closely at the top of a pond and you will usually see signs of the skin there, too. You may see little flies walking about on top of the water and not sinking. You may also see insects called pond skaters, glinding across the water.

You can also see the surface tension working if you try to make a sieve hold water. Pour only a little water into the sieve very gently. The water will fill the holes in the sieve and bulge through them, but should stay there if you have been careful. Now try putting water in the sieve again, but mix a few drops of washing up liquid with water first. What happens this time?

When you go camping, you have to be careful not to break the surface skin of raindrops on the outside of the tent. If you touch the tent inside, the surface skin will break and the water will come through.