Two sponge balls are shown. The magician puts one in his pocket, and the other in a spectators hand. With a magical gesture, the ball in the magicians pocket disappears, and ends up in the surprised spectators hand, that now has two balls.
The same effect is repeated for three times, in different ways.
To end the routine, the magician puts three sponge balls in the spectators hand, and instantly they transform into a large sponge cube!