Place the thumb tip loosely on your left thumb. If you have never used a thumb tip, it might seem a little unwieldy. Keep your hands moving and don't draw attention to it, the audience will not notice that you are wearing one.
Display a 1-inch sponge ball with your right hand. Give it to a spectator to check. While your spectator is checking the ball, make a loose fist with your left hand. Keep your thumb inside the fist. Grasp the thumb tip with the fingers of your left hand and slide your thumb out. Your thumb is now outside of the fist, and the thumb tip is inside the fist and held in position by the four fingers of your left hand.
Push the sponge ball into your left fist. The sponge ball slides into the thumb tip. Under the pretense of making sure the ball is definitely in your fist and has not been palmed back into your right hand, open your right hand and push your right thumb into your left fist. The spectator notices only your empty right hand. In reality, you force the sponge ball to the end of the thumb tip and then put it onto your right thumb.
Hold up your left fist and casually drop your right hand to your side. All attention is now on your empty left hand. Tell your audience that if you squeeze a sponge ball hard enough, it can vanish. Make a big show of squeezing your left fist as hard as you can. At the same time, slip your right hand into your pocket and detach the thumb tip and sponge ball. Take your right hand out of your pocket and wrap it around your left fist and squeeze hard. This is showmanship designed to keep your audience's attention on your left hand. After a few seconds, relax and slowly open your left hand to show the ball has "disappeared."