Perpetual Motion Coin Myth
Essentially two coins are held in the hand parrel to each other and a coin is held edgewise between them. The idea is that the coin in the middle should be able to take advantage of perpetual force and spin indefinately (which is where the myth part comes in).
Its not too hard to get a revolution or two but to get it spinning for longer periods is hard, if even possible. Its one of those things you hear someone's friends uncle once did it for 30 seconds straight ;-)
Its worth experimenting with different coin types for this. Some have nice grooves or patterns in them that might help with the trick, and smoother coins are also worth trying it with.
You can help the coin spin by subtly adjusting the pressure and angle on the middle coin with the top coin. Also adjusting the angles of the top and bottom coins can allow gravity to effect the middle coin and get it spinning a bit. You can also start it spinning artificially by flicking it with a finger or blowing on one side.
If you want to start practicing this an easy way to start is to place one coin flatwise on the palm of one hand and placing the middle coin in with the other hand (which also holds the top coin). If you can get something happening that way then you can experiment with ways to assemble the structure and get it spinning with just one hand.