No assembly needed.
Take one of the tubes that you made from a full sheet of paper in your right hand. Hold it up to your right eye and look through the tube, keeping both eyes open.
Now put your left hand, fingers up, palm toward your face, up against the left side of the tube, about two-thirds of the way down. Notice that you see a hole in your hand.
One eye sees a hole, the other sees a hand. Your eyes and brain add the two images together, creating a hand with a hole in it!