The visitor drops a large metal washer through the slot while watching through the window for it to appear inside the box. It has vanished, and didn't even make a sound when it went. The children shown in the photograph have just inserted a washer and are trying to see what happened to it. A few days later during a visit of an Elder hostel group I saw three senior citizens in exactly the same pose, but I wasn't quick enough with my camera. To heighten the illusion, the visitor can look upward through the front window at the bottom of the lid, and apparently see the bottom of the slot.
Nothing comes through.
The mystery deepens. The visitor can then lift the lid of the box to get the washers back. What is seen is a sloping area inside taking up about half the box. The area is cushioned with carpet so the washers don't make much sound when they drop through. At first, most visitors quickly notice that the illusion uses a mirror, but can't exactly explain how it is used.