Science Puzzle
Why Are Manhole Covers Round?
A manhole cover is a slab of cast iron that often weighs 50 kilograms or more. It sits over a shaft that can drop several metres to the sewers, resting on a narrow lip just inside the rim, so the cover is only a little wider than the hole.
Look down almost any street and you'll find the covers are round. Why round, rather than square?
The Answer
A round cover can't fall down its own hole. A square one can.
A square is wider across its diagonal than across its sides: about 1.4 times wider. Lift a square cover, turn it up on its edge and line it up with the hole's diagonal, and it drops straight through. A 60 cm hole has a diagonal of about 85 cm, which is plenty of room for a cover only 62 cm wide.
A circle measures the same across whichever way you turn it. Whichever way a worker tilts or twists a round cover, it's always 62 cm wide and the hole is never more than 60 cm, so there's no angle that lets it slip through. Nobody gets a 50 kilogram lid on their head at the bottom of the shaft.
Round covers have a couple of handy extras too. You can roll one along instead of carrying it, and you never have to line it up to drop it back in place. And the circle isn't the only shape that's the same width every way. A rounded triangle, called a Reuleaux triangle, works as well, and so do the seven curved sides of the British 20p and 50p coins, which is why a coin slot can measure them whichever way up they go.
The principle: Shapes of constant width. A circle is the same width in every direction, so a round lid can never be tipped through a slightly smaller round hole, while a square's diagonal is about 1.4 times its side and gives a square lid a way through.