Purchase a pre-bitten quarter from a magic store or use a metal file to remove a bite-sized chunk from a real quarter. Commercial trick quarters can cost from 10 to 20 dollars, but filing a real quarter can be quite time consuming.
Conceal the trick quarter between your thumb, forefinger and middle finger with the bitten part facing up.
Ask someone for a quarter. Show everyone that the quarter is intact, then quickly place the quarter under the trick quarter and move the trick quarter up.
Bite down on the trick quarter and show everyone the coin with the bitten chunk.
Blow on the bitten quarter and quickly switch it with the real quarter.
Give the real quarter back to its owner.