The amazing floating coin!

No, this isn't a physics class trick (or maybe it is?)
On my 2002 holiday in Indonesia I got a 100rp coin as change from an Internet cafe (A hundred rupiah is about one cent, so even for elementary schools it makes for a cheap trick (which was something else I was offered down there :)). It weighed very little, which kinda surprised me since most norwegian coins are heavy kopper-nickel/zinc alloys. It was so light I bet that it would float in water. And I won.

Allright then, so it's not lighter than water, but surface tension is still water! Indonesian water was much less pure than norwegian water (some places salt), so I had some problems making it float again. You can see that it pulls the surface down about two-three milimeters, and in case you took experimental dance class instead of science (sigh), here's the reason why this surface tension is. (if you're a moron, don't worry. I explained it as easily as I can without hand puppets.)

A water molecule is made of two hydrogen atoms and one oxygen atom. These atoms stick together to form a molecule because they've got shared custody of electrons. Oxygen, however, has a higher electronegativity than hydrogen, and thus grabs more than it's fair share of electron time. This causes it to be slightly negativly charged, while the hydrogen (which has less electron time) is positivly charged. Like magnets, the positive hydrogen part of the water molecule is drawn to the negative oxygen part of other water molecules. This inter-molecule bond is called a hydrogen bond. On the surface molecules, there are less molecules around (there are none "on top" of them), so the exhibit more force on their peers. When you see the astronauts squirting out a big bubble of water, this is why it sticks together instead of dispersing like a ball of smoke. It's also this that allows you to fill a glass of water clearly over the edge. And it's what allows certain bugs to walk on water (like spiders, beetles and Jesus).