Algae are simple living things that make their own food from sunlight, like plants do, but they're not actually plants. Most algae live in water, from tiny pond scum to giant kelp forests in the ocean. Some are so small you need a microscope to see them, while others grow dozens of feet long.

You've probably seen algae even if you didn't know that's what it was. That green film on an aquarium wall? Algae. The slimy green coating on a rock in a stream? Also algae. The bright green pond that looks almost like paint? Millions of microscopic algae growing together.

Algae are incredibly important. They produce much of Earth's oxygen through photosynthesis, just like trees and other plants. Tiny ocean algae called phytoplankton form the base of the ocean food chain: small fish eat them, bigger fish eat those fish, and so on. Without algae, ocean life as we know it couldn't exist.

Scientists don't classify algae as plants because they lack roots, stems, and leaves. Instead, they're their own diverse group of organisms, ranging from single cells to complex seaweeds. Some algae even team up with fungi to form lichens, those crusty patches you see on rocks and tree bark.