This interactive exhibit at the science museum has a name:

Why are most manhole covers round? (Also commonly referred to as: Manhole Cover Safety Test, Curve of Equal Width Experiment)

Game Principle & How to Play

1. The exhibit features five types of manhole covers—round, square, rectangular, elliptical, and Reuleaux triangle—along with corresponding openings.

2. Try inserting each differently shaped cover into its opening, both diagonally and vertically.

3. You’ll discover that: round and Reuleaux triangle (curves of equal width) cannot fall through no matter how they’re oriented, whereas squares and rectangles will slip right into the opening when tilted.

4. Key educational insight: The geometric safety principle behind curves of equal width, explaining why circular manhole covers are so prevalent in urban settings.
   Based on this concept, we’ve redesigned a similar educational toy with enhanced usability and added storage functionality.
   The set includes shapes such as circle, triangle, pentagon, hexagon, and octagon.
   A central hole allows children to easily pick up and place the pieces.