Mystery boxes are a great tool to practice scientific reasoning skills and to introduces students to the power of scientific models. Students develop hypotheses about the internal structure of a mystery box, and come up with ideas how to test their hypotheses through indirect observation. In this case, a 5 mm steal ball is enclosed in the box, students observe the sound is makes when moving the box. Students then modify or replace their models if they do not explain all their observations. As a second level, give them a small neodym rod magnet (such as S-05-14-N) for more accurate observations.
We provide 3D designs for 11 different internal structures covering different difficulty levels. We know it’s temping, but NEVER open the box, that’s not how science works. If you cannot resist the temptation, glue the lid and the base together (that's what we did). You don’t have a 3D printer? Check out cardboard or pipe alternatives, e.g. developed by the Perimeter Institute.