Bite Out Quarter
A coin is borrowed from a spectator, and a piece is bitten from the coin.
The coin is shown in full view with the piece missing. The magician draws attention to the bitten coin and spits the missing piece back at the coin. Instantly, the coin is restored to its original state.
The coin can then be handed back to the spectator.
Handing out the coin requires a simple sleight of hand switch.