The question of why manhole covers are typically round was made famous by Microsoft when they began asking it as a job-interview question.[6][7] Originally meant as a psychological assessment of how one approaches a question with more than one correct answer, the problem has produced a number of alternate explanations, from the pragmatic ("Manhole covers are round because manholes are round.")[6] to the philosophical. Reasons for the shape include: A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.) Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape. Similarly, it is easier to dig a circular hole and thus the cover is also circular. The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe. Circular covers do not need to be rotated to align them when covering a circular manhole. A round manhole cover can be more easily moved by being rolled. A round manhole cover is cheapest to manufacture relative to other shapes because it requires the least amount of metal to cover an opening wide enough for a person to get through. Most manhole covers are made by a few large companies. A different shape would have to be custom made. If a manhole cover were to be displaced by a car or anything else, it could easily slip back into position and would not have to have any angles matched up. If a manhole cover were to be displaced it would not pop or damage the tires of an automobile. Other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire may be unique in the U.S. for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local foundry, because they are not large enough to meet modern safety standards and a manufacturer for larger triangles cannot be found.[8] Some manhole covers in Hamilton, Bermuda are triangular, and hinged. Some triangular water-main covers also exist in San Francisco.