The girls got to see the "skin" on top of water today. First we filled the cup ever so slowly so that they could see how the water bulged over the top of the cup - held in place by the skin.
Then we even manged to get a needle to float on top of the skin. A steady hand is needed as the needle naturally wants to sink. Then we took a toothpick and put some dish washing liquid on it. As soon as the toot pick touched the water the skin was made too thin (from the dish washing liquid) and so the needle instantly sank.
Then the girls made their own submarine. When filled up with water this bottle sinks. But when air is put in it comes up to the surface.