A beaver’s teeth never stop growing.
A beaver's teeth will grow continuously throughout its life. Otherwise, they'd be worn down to nothing from gnawing on wood. However, a beaver must regularly "file down" its teeth in order to avoid them growing into their brains or otherwise becoming unusable. Yikes.
Beavers use their teeth on trees and branches, which they use to construct dams that will block up small streams to create large ponds. They also use logs, mud, and branches to build domelike homes strategically located in the middle of these ponds, with underwater passages the only means of ingress or egress. While these large rodents waddle awkwardly on land, they swim rather gracefully. Ponds provide a quick escape route from predators, while their domes provide an added layer of security for their families.
Image: Robert McGouey/All Canada Photos/Corbis