The Effect: The magician holds and ordinary Styrofoam cup in their hands, and it floats when they let go!
The Secret: This trick was demonstrated by Criss Angel and it will definitely impress your friends.
Before you start, push your thumb into the back of the cup. It's best that the cup is Styrofoam, because the audience can feel the cup and your hands for glue or tape. Once they inspect it, secretly push your thumb in from the back. Then hold the cup with both hands and pretend to concentrate. Then let go and your thumb should hold the cup up. Then grab it quickly. Repeat this as much as you like to impress your audience.
Tip: This is best viewed from the front because if your audience saw the back or side, it would reveal the secret!