This is a very impressive, yet easy to perform routine that combines a number of different magical principles.
The magician shows a small black box, opens it, and removes an even small red box.
She then opens the small red box to show a small red sponge ball inside, that totally fills the box - this ball is removed and placed to one side.
The magician then picks up the small black box, closes it up, and then places it inside the red box - so the black fits inside the red, and the red fits inside the black.
Then ... then red box is opened up, and the black box is removed. The black box is opened up to show another red sponge ball inside! This is then removed.
The red box is then closed up, and opened up again to show a small yellow ball appearing inside.
So there are now three sponge balls that have appeared from the boxes, that fit inside each other.
Finally, the first red sponge ball is picked up, placed inside the magician's hand, and then shown to change into a sponge cube!!!