OK, so it probably isn't an "official" interview question (if there are any), but I have asked it during interviews in the past. For me, it is simply a way to help loosen up the interviewer and have some fun, but if the only answer I get is "I don't know, why is it?", then that gives me the impression that the person either isn't trying to really interview, or that they just aren't very creative.
So, why is a manhole cover round? Heck if I know, but here are some good answers that I've heard over the years:
I suppose there are more, and there is probably some web site with the "correct" answer. But I like these because I can remember the individuals who have told them to me over the years. What's your answer?