If you have a PC, tablet, or smartphone, you have used a computer. But some people use billard balls, beams of light, sticks of wood, chemicals, bacteria, slime moulds, spaghetti, even black holes, as computers (although some of these only in theory). How can these things be computers? What can we do? Can they do things your smartphone can’t? And why are these people using such peculiar things to compute with, anyway?