You present this as a puzzle, mathematical curiosity, optical illusion, or magic trick. The boxes
are made from plastic and will likely last a lifetime assuming no major abuse. The boxes are just
big enough to hold a 1" sponge ball. I like to open the black box, pull out the red box, and remove
a black ball. In reassembling the boxes, I load a red sponge ball into the black box which is placed
in the red box. I mention how much it bothers me that the ball wasn't in the right spot and say I
have to fix it. Reveal the other ball and then I can go into a sponge ball routine.