A guppy is a small, colorful freshwater fish that's one of the most popular pets for home aquariums. Guppies rarely grow longer than two inches, but what they lack in size they make up for in brilliant colors: males especially can have tails that shimmer with blues, reds, oranges, and yellows, like tiny swimming rainbows.

Guppies are native to streams and ponds in parts of South America and the Caribbean, but they've become favorites worldwide because they're easy to care for and fascinating to watch. They're also remarkably hardy, able to survive in water conditions that would trouble more delicate fish. Female guppies give birth to live baby fish rather than laying eggs, and they can have dozens of babies at once, which is why guppy populations can grow quickly.

In casual conversation, people sometimes call someone a “small fish” or a guppy when talking about beginners or less experienced people in a group, similar to calling someone a small fry. A new employee at a big company might be just a guppy in a sea of experienced workers.