Wombats leave droppings shaped like cubes, and for a long time nobody knew how, because the exit is round like everyone else’s. The answer, published by Patricia Yang and her colleagues at Georgia Tech in 2021, is that the corners are made earlier, in the gut. The wall of a wombat’s intestine is not the same all the way round: some bands are thicker and several times stiffer than others, and as it squeezes, the soft stretches give and the stiff ones hold, pressing flat faces with edges between them. The cube only sets near the very end, in roughly the last twelfth of the journey, dry enough by then to keep its shape on the way out. Factories make cubes by moulding or cutting. The wombat found a third way, which is to squeeze unevenly on purpose. It is the opposite of the old saying: here is a square peg, and it passes through a round hole without any trouble, because it was square before it got there.