Iron Doughnut

A piece of solid iron in the shape of a doughnut is heated over a fire. As the iron expands, does the hole become larger or smaller or does it remain the same size?

As the doughnut expands, it keeps its same proportions; so the hole also gets bigger. This same principle is at work when an optician removes a lens from a pair of glasses by heating the frame, The next time you can't open the metal lid on a stubborn jar, heat the lid under hot water. The lid, inner circumference and all, will expand, making it easier to loosen.