Stella was out for a walk, and she noticed for the first time that all the manhole covers she encountered were round. She wondered why, and after a moment she realized that it was for a very good reason. Indeed, she realized that no polygonal shape would do. Can you figure out what this reason is? (The answer is not "because manholes are round".)
Solution
The most important reason is safety: A square or rectangular manhole cover could fall into the manhole.