Is shown to the public a plastic tablet with a red dot on one side and a blue dot on the other. The tablet is closed and asks a spectator to remember which side is the red dot. He can not guess, because when the tablet is reopened, the dots have changed color and became one black and one yellow instead of red and blue. 

At the end the audience can examine everything.

There is also a cheaper version of this routine (gimmick) available through this Link