Four in one
I did this project during my first year at my Academy. The task was to design and make a cube which would include elements inside. The last element had to block the other pieces and prevent them from falling out of the cube.
I made 2 versions of this cube. The first one was made of wood, the second one of plexiglas pieces. Inside the smallest cuboid I made a hole in which I put a copper, cylindrical shaped piece. I created a similar hole in the medium-sized cuboid, too. Both of the holes were placed in a position which allowed them to stick while the cube is set. Thanks to that, after flipping the cube upside down, the copper piece moves to the second hole, preventing the smallest cuboid from falling out of the cube.