A bur (also spelled burr) is a prickly seed case or flower head that sticks to clothing, animal fur, or anything that brushes against it. If you've ever walked through tall grass or weeds and found small, spiky balls clinging to your socks or shoelaces, those were burs. Plants produce them as a clever way to spread their seeds: the hooks and spines catch onto passing animals (or people), and the seeds get carried to new places before eventually falling off.

Common burs include those from burdock plants and cockleburs. They inspired the invention of Velcro in 1941, when a Swiss engineer named George de Mestral noticed how stubbornly burs stuck to his dog's fur after a walk. He examined them under a microscope and saw tiny hooks, which gave him the idea for a two-sided fastener: one side with hooks, one side with loops.

The word bur can also describe any rough, prickly edge, like the sharp bit of metal left on a pipe after cutting it, or a rough spot on wood. That kind of bur needs to be filed or sanded until it's smooth.