A protist is any living thing that has cells with a nucleus but isn't a plant, animal, or fungus. Scientists created this category for organisms that don't fit neatly into the other major groups of life.

Most protists are microscopic single-celled creatures living in water, soil, or even inside other organisms. Amoebas are protists that change shape as they move and eat. Paramecia are slipper-shaped protists covered in tiny hairs that beat like oars to propel them through pond water. The green scum called algae that grows in aquariums and lakes? Those are protists too, and they make their own food through photosynthesis, just like plants do.

Some protists are surprisingly large: kelp forests in the ocean are made of protists that can grow over 100 feet long. Other protists cause diseases, like the parasite that causes malaria.

Today, scientists know that “protist” is really a catch-all category for diverse organisms that happen to share a few basic features. Think of it like a drawer labeled “miscellaneous,” where you put things that don't belong anywhere else.