This one is fairly simple and very straightforward.
So, you have a bag of marbles and can no longer find anyone that "plays" marbles? Have I got a plan for you !
Step 1: Ingrediants...
One cardboard box, able to hold about 50+ marbles, and also thin enough to fit inside a medicine cabinet.
One bag of marbles, about 50 or more should be enough.
The cardboard box should be completely enclosed with a small flap on the broadest side (front or back) for the opening.
Fill the box with the marbles, close the flap, and place it up-side-down inside the medicine cabinet, carefully closing the door to hold the flap closed (if your chosen box has no flap that extends to the broad side, you can cut one, as illustrated, so you don't have to turn THIS box, bottoms up. . .
Step 2: Execution...
Now that you have the trap setup. It is ready for your snooping relative or friend to come in and open the cabinet door.
This is really great if you know who might do this, and so standing outside the bathroom door, you will hear the marbles hitting the sink; you can yell in "Is everything ok in there?" LOL