If you’re in the habit of looking down at your feet as you walk, you may have noticed that most manholes, and their covers, are round. Or perhaps you’ve been in a job interview where your prospective employer asked this question (which apparently happens enough that it was included in the third edition of The Complete Idiot’s Guide to Job Interviews). He wants to know about your ability to think on your feet, and isn’t so much interested in the correct answer, but it is a good question. Of all of the potential shapes for manholes, why was a circle chosen?

There are a number of reasons, according to Larry Scheckel in his book Ask A Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works. Manholes are round because “it is the best shape to resist the compression of Earth around it.” Round shapes are easier to manufacture than square or rectangular shapes, and because manhole covers are heavy, being round makes them easier to move from place to place (just roll them!). As an added benefit, workers don’t need to line up the covers with any angles, making round covers easy to slip into place.

But perhaps the biggest reason that manhole covers are round is that round covers can’t fall through a circular opening. “For all manholes, there is a ‘lip’ around the rim of the hole, holding up the cover, which means that the underlying hole is smaller than the cover,” Scheckel writes. “A round manhole cover can’t fall through its circular opening, because no matter how you position it, the cover is wider than the hole. But a square, rectangular, or oval manhole cover could fall in if it was inserted diagonally into the hole,” which would be bad news for unobservant pedestrians and drivers alike.