A British fifty-pence coin has seven corners, and it still rolls. Its sides aren’t straight: each one is an arc drawn from the corner opposite, which makes the coin a curve of constant width. Measure it across in any direction and you get the same number, and that was the point when it arrived in 1969, because a vending machine judges a coin by its width, and a coin that measured differently depending on how it fell into the slot would be rejected half the time. There are infinitely many shapes like that, and a circle is only the plainest of them. What the circle has that the others don’t is a middle that stays put. Roll a fifty-pence piece between two rulers and they glide along perfectly flat, but its centre wobbles up and down as it goes, which is why nobody has ever put one on an axle. A wheel needs more than constant width. It needs a centre that stays where it is.