These are a pair of boxes, one black and one red, (approximately 1-1/4 inches in size) each having their own cover.
The magician opens the black box and inside is a red box. The red box, which is smaller, is removed, and the magician closes the black box.
Now he puts the black box into the red box! Now the black box is smaller than the red!
Repeat this as often as you like.
Boxes made from heavy gauge plastic for long life.
Complete with instructions.