A flatworm is exactly what it sounds like: a worm with a flat body instead of a round one. If you've ever lifted a rock near a stream and seen a tiny, ribbon-like creature gliding along, you've probably spotted a flatworm. These simple animals look like they've been squished, with bodies so flat and thin you can sometimes see right through them.

Flatworms come in thousands of varieties. Some live freely in oceans, lakes, or damp soil, hunting for tiny prey. Others are parasites that live inside other animals (including humans). The most famous parasitic flatworm is the tapeworm, which can grow incredibly long inside an animal's intestines.

Despite their simple appearance, flatworms are fascinating to scientists. They have incredible regeneration abilities: if certain flatworms are cut into pieces, each piece can grow into a complete new worm. Some flatworms even have primitive eyes that detect light, though they can't see images like we can.

In biology class, you might study flatworms as examples of simple animals. They don't have lungs, hearts, or blood vessels. Instead, they absorb oxygen directly through their flat bodies, which is why they need to stay thin. Their flatness isn't a weakness but an elegant solution for survival.