Multiplying Sponge Feet

A small foot? is placed into a spectator's hand.

When they later open their hand, now there are two feet!

One of the?feet is placed into the magician's pocket and the spectator closes their hand around the other.

Again, when they open their hand, another?feet appears!

The two?feet multiply again, becoming four!

As a finale, the four?feet all change into one large feet.