As we drive on the road or walk across the street, we always see manholes covered in circle-shaped steel. As I ask myself, why are manholes covered in round-shaped steel rather than square? A square cover could fall into its hole, injuring something or getting lost under the hole. Should you hold a square manhole cover almost vertically and turn it a little, it will easily fall into its hole. As for a circle, it has the same diameter in all directions. This and the slight recess in the lower part of the cover prevents a circular cover from falling in, no matter how you hold it. If you try to remove the cover from the hole, a circular cover can be rolled to move a short distance while a square cover would require a trolley or two person to lift it up.