Colour Tap

The magician shows four small coloured squares of cardborad and puts them on the table, inside four small envelopes. A colour is mentally selected and the magician starts tapping on the envelopes and, when the spectator calls STOP, the magician stops tapping and shows that the card inside the final tapped envelope is the correct selected colour. Complete with instructions which have BCM/TRICKS London, W.C.1 on them.