Cilia is Latin for "eyelash." Common in single-cell organisms, this hair-like structures wave to move a cell around, or to move something around the cell. Some tissues in the human body also have cilia.

In the body, cilia are responsible for protecting a person from germs in the lungs and for pushing an ovum down the fallopian tube, among other tasks. Similar to cilia, flagella are longer such hairs, usually found in ones or twos, such as the "tail" of a sperm. They share many characteristics with cilia, but they also occur on prokaryotes, which are organisms with cells that do not contain a nucleus.

Single-celled eukaryotes, which are organisms with cells that do have a nucleus, often use cilia to move through liquid. These organisms are surrounded by a cytoskeleton, made of protein filaments that allow the cell to hold its shape. A cilium attaches to the cytoskeleton of the cell with a basal body, the way a root attaches hair to our skin.

The rhythm of the waving cilium is controlled by centrioles, which are organelles located inside the cell wall. Mitochondria, other units inside the cell, provide adenosine triphosphate (ATP), a source of cellular energy, for the cilia. The ATP directs the chemical kinesin to bind to certain parts of the cilia that control their movement. Thus, the cilia are able to "beat" or "swim" their way through viscous liquid.

The structure of a cilium is much like a tube, and its long fibers are called microtubules. These microtubules often pair up to form doublets, which in turn form a ring. The cross-section of doublets of microtubules looks like a figure eight, since the two microtubules stick together along a line. Nine doublets form the larger ring in what is known as the 9-2 pattern. When kinesin binds to one side of the doublets and not the other, the cilium flexes and curves, similar to the way our skeletal muscles contract.

Some eukaryotes that use cilia and flagella to move are also found in ferns, algae, bacteria, and inside many animals. This adaptation originally allowed independent cellular creatures, like paramecia, to move around in search of food, rather than wait until food came to them. Since then, cells as part of larger systems continue to use cilia to great advantage.