A burr is a prickly seed case that sticks to clothing or animal fur using tiny hooks. If you've ever walked through tall grass or weeds and found spiky brown balls clinging to your socks or shoelaces, you've met a burr. Plants evolved burrs as a clever way to spread their seeds: animals (or people) brush against the plant, the burrs latch on, and later fall off somewhere far from the parent plant.

Burrs inspired one of history's most useful inventions. In 1941, a Swiss engineer named George de Mestral noticed burrs stuck to his dog's fur after a walk. Examining them under a microscope, he saw their tiny hooks and realized he could create a fastener that worked the same way. This led to the invention of Velcro, which uses the same hook-and-loop principle that burrs have used for millions of years.

The word burr can also describe a rough edge left on metal or wood after cutting or drilling. Woodworkers and machinists carefully remove these burrs to make surfaces smooth and safe. Sometimes people use burr to describe pronouncing the letter “r” with a rolling or trilling sound.