The magician opens a red box and reveals a black box inside. Inside the black box a red sponge ball can be seen. The red sponge ball is removed, now the red box is closed and put in the black box (the box which was on the outside before is now located inside the other box).
The boxes are opened again, now inside the innermost box, a blue sponge ball is being found. Again the boxes are closed (now red box is on the outside again, and the black box is inside). After opening the boxes for the last time, a yellow sponge ball is revealed.
Now the 2 boxes can be shown to be exactly the same size.
It is possible to hand out the boxes, the yellow, blue and Red sponge ball.