Suppose you are walking along a deserted sandy beach on a tropical island when you come across a magniﬁ cent pile of sand. At each corner is a square tower, decorated with rows of shells, and all round is a moat with a ﬂ at stone for a bridge, neatly attached to threads of seaweed for pulling it up. What will you conclude? It’s a sand castle of course. And somebody must have built it.