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 copper-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, which is the magic of surface tension. This is the same property that allows you to fill a glass of water clearly over the edge, and what allows certain creatures to walk on water (like spiders, beetles and Jesus). Wikipedia has more on surface tension than you'll ever want to know!