Poop comes in all shapes and sizes, from tiny pellets to rounded piles. But wombats—furry mammals that live in Australia—are the only animals known to make cube-shaped poop. Engineer Patricia Yang recently found out how.

She analyzed the intestines of dead wombats. Intestines are a tube-shaped body part where food breaks down and solid waste forms. Yang found that the intestinal walls of wombats stretch unevenly compared with those of most other mammals. Stiff areas follow stretchy areas. As waste moves through the intestines, the tighter parts squeeze the poop, forming flat surfaces on each side, resulting in cubes.