Why are fire hydrant caps pentagonal?

Answer 1

The main reason is so that vandals with a regular wrench cannot turn a square nut and steal water. But since this is about geometry it is probably since the nut is soft and made of brass that a square wrench would round the nut and damage it quickly. Having a pentagon shape gives more area for the wrench to grip and a triangle is the strongest geometric shape so the force is more distributed from the points to the center of the shaft. That lets you turn it with force without stripping or damaging the nut.