Fill an ashtray with water.
Bet someone that you can get all of the water into a beer glass without moving the ashtray.
Place a stack of six quarters in the middle of the ashtray. Make sure the top of the stack is well above the water.
Place four unlit matches on top of the quarters.
Light the matches and quickly place the glass upside down over the matches, so the rim of the glass is submerged in the water.
Amaze your friends with the results: The burning matches will use up the oxygen in the glass, and the water will be sucked into the vacuum created by the matches.