Here's a typical job interview question from McKinsey to rack your brain: why is a man hole cover often round as shown here?

Here are my possible answers:

1) If the man hole cover had been a square cut, it will drop into the manhole easily through the diagonal space;

2) When taken out, one can easily roll the manhole cover, which is too heavy for hand carry, on the ground;

3) The drill for creating the manhole is round;