Ask a child to draw some poo, and the shape will invariably be the same: a coil, broad at the base and pointy at the top, similar to a spiral swirl of soft-serve ice cream. In fact, the often-used poo emoji has this exact shape, as do most actual mounds of faeces found in nature. Exceptions occur, though: in particular, in the faeces of some worms, that extrude their excrement ‘upside down’ from the ground. As it turns out, there is remarkable physics behind these differences in poo shapes.

Already in 1881, Charles Darwin marveled at the neat spiral-shaped faecal castings of earthworms. However, he never understood why these faeces take their fascinating, tower-like shapes. Now, physics has provided the answer—and it explains both Darwin’s original observations and the shape of the modern poo emoji.

Most animals defecate downward. As their faeces coil, the pile grows higher and the fall height decreases, making each successive coil smaller. This creates the characteristic pointy mound—precisely the shape of the poo emoji. Lugworms, however, defy gravity by extruding their faeces upward. A new study by physicists Mehdi Habibi (Wageningen University and Research), Neil M. Ribe (CNRS/Université Paris-Saclay) and Daniel Bonn (University of Amsterdam), published in Nature Communications this week, reveals something remarkable: the lugworms operate in a previously unknown physics regime where coil size is independent of height – exactly as Darwin observed. The coil radius stays constant, creating uniform spirals rather than tapered mounds.

The researchers found that both types of faeces obey the laws of elastic rope-coiling theory – as the name suggests, a mathematical description of how ropes and other materials coil. It turns out that the only differences that cause the two types of shapes are the stiffness of the material and, most importantly, the direction of gravity compared to the direction of extrusion. The observations – and their mathematical underpinnings – do not just hold for faeces: the researchers tested their ideas on lugworms, on extruded pea dough – a mix of chickpea flour and water similar to the wet sand of the worm poo – and even on pasta, and confirmed that different directions consistently lead to the characteristic different shapes. Darwin may have been sad to learn this: evolution, it turns out, doesn’t design waste disposal. Physics does.