A leech is a small worm-like creature that lives in water and attaches itself to other animals to drink their blood. Leeches have suckers at both ends of their bodies, and they use one end to latch onto a host (like a fish, frog, or even a person wading through a pond). Once attached, they feed on blood until they're full, then drop off. While this sounds creepy, leeches don't usually take enough blood to seriously harm their hosts, and their bite contains a substance that numbs the skin, so you might not even feel it happening.

For centuries, doctors actually used medicinal leeches to treat patients, believing (often incorrectly) that removing blood would cure diseases. Today, doctors occasionally still use leeches in specialized surgeries, because leech saliva contains chemicals that improve blood flow and help tissue heal.

The word also describes a person who takes advantage of others without giving anything back. If someone constantly borrows money but never repays it, always expects help but never offers any, or takes credit for other people's work, you might hear them called a leech. Like the animal, a human leech attaches themselves to others and drains their resources, energy, or goodwill. Nobody likes having a leech around, whether in a pond or in their life.