A lemming is a small, furry rodent that lives in the Arctic tundra of northern Europe, Asia, and North America. Lemmings are about the size of a hamster, with thick fur that keeps them warm in freezing temperatures. They spend much of their time burrowing through snow and building tunnels, munching on grasses, moss, and roots.

Lemmings became famous because of a persistent myth that they jump off cliffs when their populations get too large. This simply isn't true. In reality, lemmings do sometimes migrate in large groups when food runs short, and some accidentally fall while trying to cross rivers or lakes, but they don't intentionally jump off cliffs.

Today, people use lemming as a metaphor for someone who mindlessly follows the crowd without thinking. If someone says “Don't be a lemming,” they're warning against blindly doing something just because everyone else is doing it. Ironically, this usage comes from that false myth about the real animals. Real lemmings are actually tough, clever survivors in one of Earth's harshest environments.