A flagellum (pronounced fluh-JEL-um) is a long, whip-like tail that certain microscopic organisms use to swim through liquid. The plural is flagella.

Picture a tiny bacterium invisible to your naked eye, surrounded by water. It waves its flagellum back and forth like a propeller, pushing itself forward through the liquid. Some bacteria have just one flagellum, while others have several sprouting from their bodies. Without these remarkable tails, these organisms would be stuck drifting wherever currents took them.

Scientists discovered flagella when microscopes became powerful enough to reveal the hidden world of single-celled life. What looked like simple dots under early microscopes turned out to be complex organisms with their own swimming equipment.

Flagella aren't just for bacteria. Some algae use flagella to move toward light. Even the parasite that causes malaria uses a flagellum during part of its life cycle. These tiny biological motors have been around for billions of years, helping microscopic life navigate through watery worlds long before anything with legs, fins, or wings existed.