Ball changes color
Step 1: Show your audience a small, green ball.
Step 2: Then cover the ball with a handkerchief and have a volunteer from the audience hold it.
Step 3: Instruct the volunteer to hold the ball with the handkerchief over a clear cup.
Step 4: The volunteer then releases the ball into the cup.
Step 5: You remove the handkerchief from the cup, and what do you see? Not a green ball, but a red one!
Step 6: How did that happen? At the start you had two balls, identical in size. The red ball was simply hidden in your hand.
Step 7: At the right moment you switch the balls. And then while everyone is watching the volunteer, you drop the green ball into your lap or pocket.
Step 8: What could be simpler than that?