Plankton are tiny living things that drift through oceans, lakes, and rivers, too small or weak to swim against currents. Most plankton are so microscopic you'd need a microscope to see them, though some jellyfish that drift with ocean currents are sometimes counted as plankton, too.

Scientists divide plankton into two main groups: phytoplankton are microscopic organisms that use sunlight to make food (like underwater plants), while zooplankton are tiny animals that eat the phytoplankton or each other.

Though individually tiny, plankton might be some of the most important organisms on Earth. Phytoplankton produce a large share of the oxygen we breathe, and they form the foundation of many ocean food chains. Small fish eat plankton, bigger fish eat those small fish, and so on up to sharks and whales. Some of the largest animals on Earth, like blue whales, survive by filtering millions of tiny plankton from seawater. Without plankton, ocean life as we know it couldn't exist.

When you look at the ocean, remember: that seemingly empty water is actually full of invisible plankton, quietly helping keep our planet alive.