The impossible dovetail puzzle works in two ways:
- As you can see in the picture, because of the design, it is only possible to open it sliding on its diagonal.
- But there's more, there is a small steel ball that locks the box. It is held by two magnets. One of them is in the upper part, and the other in the lower part (the one with the deeper hole). When the ball is on the upper part (held by the magnet), it locks the box, so it can't be opened. In order for you to open it, you need to tap the box to make the ball go from one magnet to the other. Let's say that, to be able to open the box, the ball has to be inside the deeper hole (held by the magnet).
You'll only need to glue two magnets on the bottom of the holes like in the picture. Then insert a ball in the deeper hole and slide the top part.
There is an optional step. You can insert a second ball in the other hole but without magnets. This ball will move freely when the box is closed making your friends and family go nuts trying to figure out how to open it.