I was amazed while doing a Google image search to come across a picture of a coin floating in a cup of water. I knew paperclips could float, but never thought coins would!
I immediately filled up a container with water and began testing every american coin I had around the house to see if I could get one to work. I didn't believe they would, but thought it would be mind blowing if one actually did.
After many failed attempts, I went back to the picture and studied it in more detail to see if it was a hoax or not.
On close inspection, and with a little more research, I learned the coin was actually a 1 YEN coin from Japan, and was made of aluminum. This made it nearly 3 times lighter than any of my other American coins, and explained why it could float, and the others couldn't.
We had some Japanese exchange students living with us over the years, and I got some 1 YEN coins to experiment with, and they worked no problem.
Pretty cool trick, and I understand it will work with currencies from other countries as well, so long as the coins are made of aluminum!