The magician hands over a disc printed with colors, letters and numbers, then turns his back. The spectator mentally chooses a number and, by spinning the disc, rotates it by as many positions as the chosen number. Then he stops, looks at the color he has landed on (blue for example) and turns the disc over (or hides it under the table) so that the magician cannot see it. The magician shows his prediction that will be 100% correct!
The effect can then be repeated a second time, but this time using letters. Needless to say that the magician's prediction will be correct!