This is not a trick, but a "scientific experiment" made to look like a trick. Tape a string to a penny and then suspend it over the palm of a girl. You will find that it swings slowly over her hand in a circular motion. But when you do the same thing over the hand of a boy, it will swing straight up and down over his palm.

SECRET: One scientific explanation is that the movement of the coin is unconsciously controlled by muscular vibrations. Don't try to make the coin swing by moving your arm. Just tell your audience that you're thinking "circle" over the girl's palm and "straight line" over the boy's. It will really operate itself and create a great sensation.