A stag beetle the size of a fingernail stands on the ground with its legs spread wider than its own height. An elephant as tall as a room, on the other hand, towers above the ground like a tower. Why is that? Both animals walk on the same ground, but they see it differently. For a beetle, each pebble is a boulder, while for an elephant, it's a grain of sand.

The roughness of the ground is like a coastline: the closer you look, the more details you see. For a small animal, the ground is full of bumps and pits, and to avoid tipping over, it needs a wide base, like a table with splayed legs. For a large animal, the ground is smooth, and it's more important to be tall so it can take long strides. This rule doesn't depend on how many legs an animal has or how it's built: it works for both an ant and a giraffe.

This discovery explains why body shape evolved in similar ways across very different animals. Not kinship and not chance, but simple physics of walking on uneven ground: that's what made the elephant tall and the beetle wide.