A borrowed and marked coin is covered by a handkerchief and dropped into a small glass of water. When the handkerchief is removed, the coin is seen to have vanished completely. The borrowed and marked coin is later produced from your pocket. The water is poured out, and the audience can see right through the clear glass.
Instead of producing the vanished coin from your pocket, you could also re-produce it from an Orange of other fruit, or a Ball of Wool, or special magicians props like a "Quarter Go" Coin Box or Nest of Boxes