Step 1: Produce the Sponge
Begin with your hands casually in your pockets. Use your left hand to point to a spectator's elbow and say "Hey what is that there? Behind your elbow!" When they look to their elbow, use your right hand to secretly grab the ball out of your right pocket. Keeping a closed fist, reach behind the spectator's elbow with your right hand. Slowly open your hand, giving the illusion that the sponge magically appeared there!
Step 2: Vanish the Sponge
To make the sponge disappear, you will perform a sleight called the "Retention Vanish". To perform the "Retention Vanish", hold the sponge in your right hand, palm-up, covering the ball with your right thumb. Hold your left hand open, palm-up. Turn your right hand palm-down and place it onto the palm of your left hand (make sure not to let go of the ball with your thumb). Close your left hand and pretend to grab the sponge, then remove your right hand while secretly holding the ball. Allow your right hand to fall to your side and hold your left hand up, bringing attention to it. Slowly open your left hand to show the ball has indeed vanished, then produce the ball again using the same method in "Step 1"
Step 3: Multiplying Sponge
Perform the "Retention Vanish" once again using the method in "Step 2", then pretend to place the ball into your left pocket. Produce the ball in your right hand once again using the method from "Step 1", while secretly grabbing one of the sponges out of your left pocket. Remove the sponge from your left pocket keeping it secretly in in your left hand. Quickly bring both hands together and produce the second sponge, pretending to split the first sponge in half.
Step 4: Jumping From Hand to Hand
Hand both sponges out to be examined by two different spectators. Grab one sponge and perform a "Retention Vanish", pretending to hold the ball in the left hand, but secretly keeping the ball concealed in the right hand. Then use the right hand to grab the other sponge so now you hold two in the right hand. Slowly open the left hand, showing that the sponge has vanished, then open the right hand showing the sponge has magically jumped to the right hand!
Step 5: Jumping to the Spectator's Hand
Using the same method in "Step 4", perform the "Retention Vanish" so that you hold two sponges in the right hand. Pretend to hold a sponge in the left hand and hand the two sponges concealed in the right hand to a spectator. Tell them to keep their fist closed tightly so that they don't see or feel that you actually gave them two sponges. Slowly open the left hand, showing that the sponge has vanished, then ask the spectator to open their hand showing the sponge has magically jumped to the their hand!
Step 6: Multiplying Finale
While the spectator is amazed that the ball actually jumped into their hand, take a step back and casually put your hands back into your pockets. Secretly grab the two sponges in your left pocket, then, after a few seconds remove both of your hands, keeping the two sponges concealed in your left hand. Grab the two sponges from the spectator and quickly bring your two hands together to roll the sponges up into a ball. Ask another spectator to hold out their hand and hand them all four sponges. Tell them to imagine their hand getting heavier and ask if they felt anything. Tell them to slowly open their hand and to their surprise, four sponges will magically jump out of their hand!