The key word with any magic trick is misdirection. A common trick that amateur magicians learn is the multiplying ball trick. This trick uses sponge balls as the main prop. These are ideal for the trick because they are easily compressed. The compression hides the balls from the view of the audience until they are ready for the reveal, or prestige. This trick requires a medium amount of set-up and a lot of misdirection to prevent the audience from knowing where the balls are coming from.
Prepare all four balls that will appear in the finale. One ball will be placed in the magician's mouth immediately before the trick. One will be placed in the pocket of the dominate hand. One will be placed in the off-handed pocket. One will be placed in the back belt loop.
Develop a cough before going on stage. The cough is crucial in performing the first production of the first ball in the magician's mouth. If you haven't been coughing all day, it will be readily apparent that this is how you are creating the first ball.
Have the palmed ball in your mouth and when you cough, project the ball into your hand. You may want to lean your head away from the audience whenever you cough, to protect the angles. Remember not to entirely drop the cough, or it gives away the trick.
Present the first ball to the audience. One of the most dramatic ways to do this is to hide the ball after production. Place your palm and the ball on the table and rub. As you release pressure, the ball will appear to materialize.
Give the ball to an audience member to examine. You may choose to sit down on a chair to hide your hands slipping into your pockets. Grab one of the balls in your pocket and palm it the same way as before, so that it is squeezed down. Wait for the crowd murmur to die down.
Explain to the audience that you will now make the ball disappear. Make a tube shape with the hand palming the second ball. Squeeze the first ball into the tube. You now have two balls hidden in this hand. Repeat the technique you used to produce the first ball. When you lift your hand, act as though something has gone awry, because you now have two balls instead of none.
Ask a different audience member to see if he can figure out what's wrong with your balls. Take a seat again, and this time palm the ball in your other pocket. It should live on the heel of your hand in a crook between your thumb and index finger. This is a tricker production, as it will be in nearly full view of the audience.
Take back each ball and place it between your index finger and thumb, one in each hand. One hand will have one ball and the other will have the palmed ball plus the visible ball. Extend your three available fingers to help hide the palmed ball.
Explain to the audience you will now make both balls disappear. Bring your hands together, creating a "wall" with your six fingers that are not holding the balls. Use the thumb and index finger on each hand to press the balls into your cupped hands, while simultaneously releasing the third ball into the mix.
Press your hands together to simulate combining the balls, and repeat the process you've used before to produce the balls. This time it will be slightly modified, as you will want to press both hands into the table. When you release, act more frustrated that you now have three balls instead of none.
Explain to the audience that you will have to have a volunteer help you vanish these balls because your own energy field is messed up. When the volunteer joins you on stage, start by having her take each ball, squeezing it tight in her hand. Pick up the first ball with your thumb and index finger and place it in the volunteer's palm.
Use your hand to close her hand and tell her to hold on tight. Allow the volunteer to release, and add the second ball into her hand. Repeat the process you did with the first ball. The third ball is where the sleight of hand comes in.
Scratch your back. This is the misdirection that will allow you to palm the belt loop ball. This is best done while the volunteer is squeezing the two balls. Use the same technique from the third ball production to hide the fourth one from the audience. Place the third and fourth balls in the volunteer's hand. The balls should be well hidden because you are helping her cup her hand as she squeezes. Don't let the volunteer release until you are ready for the reveal.
Pick up the magic wand, and wave it over the volunteer's clasped hand. Recite some mumbo-jumbo. Announce to the audience that you have made all three balls vanish. Instruct the audience member to release her grip. Much to your chagrin, there will now be four balls.
Remember, the moments before the reveal are more important than the results of the trick. The audience wants a story, so be sure to set up the trick.
Figure out how you want to get rid of the balls at the end of the trick. You may take a humorous approach and make them "vanish" by throwing them over your shoulder, or you may want to work another sleight-of-hand trick to make them vanish.
This trick will not go over well if the audience sees you palming the balls. Practice in front of a mirror or, better yet, videotape yourself doing the trick until it appears seamless.