Burrs have hooks or spines on their surface designed to catch onto the fibers of clothing or fur. These hooks or spines are usually curved, barbed, or serrated, which makes them difficult to detach. When an animal or a person moves through the weeds that produce burrs, the burrs are pulled loose from the plants and attach to the clothing or fur. As the animal or person moves away from the area, the burrs are carried along with them, allowing the plant to disperse its seeds and propagate in new locations.