To set up this trapdoor system, prepare a two-tier cardboard box structure with an upper deck and an enclosed lower basement cavity. Cut a one-inch square drop hole in the floor of an upper paper towel roll track. Cut a slightly smaller cardboard flap, taping one edge over the hole with a strip of flexible masking tape to serve as a swinging hinge. Glue a small metal washer to the rear underside of the flap to act as a counterweight that holds the door closed until a heavy glass marble rolls over it. Position a second, hidden paper towel tube directly beneath the drop hole at a reverse angle, routing the lower track out through an exit archway in the front of the box. Test the sensitivity of the trapdoor by adjusting the washer weight so the door drops smoothly under the marble’s mass and swings closed immediately after passage.