Magic Blood by Trevor Duffy

The magician asks the pain threshold of a spectator. He then declares that he would like to test that threshold and places a drop of "antiseptic liquid" on the spectators arm. The magician then pulls out a knife and slices the spectators arm. The spectator feels no pain, but their arm is bleeding! Before the spectator can muster up the fear to faint, the magician wipes the wound clean and reveals that no harm has been done to them! This 60 second gag will really get the adrenaline flowing!