EFFECT
David Blaine borrows a coin from the spectator he then puts the coin in his mouth and bites half the coin off. He show the half bitten coin to the audience and then blows on the coin and it magically reappears.
METHOD
This is a very easy trick to do, but it does require a special coin. The special coin has been designed to fold in two places. The front of the coin looks normal from distance, but if you look closer you can see two cuts in the coin. If you fold one of the coin flaps over it would look as though the coin has been "bitten" in half. When you let go of one of the flaps the flap springs back to it's original place so it looks like the coin has been restored.