Cilia is Latin for "eyelash." These tiny protuberances on single-cells organisms wave-like hairs to move the cell around or something around the cell. They are responsible for protecting us from germs in our lungs and pushing an ovum down the fallopian tube. 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.

Eukaryotes, single-celled organisms, use cilia to move through liquid. 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, located inside the cell wall. Mitochondria, other units inside the cell, provide ATP (adenosine triphosphate) for cilia, a source of cellular energy. 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 extended follicles in turn form a ring out of doublets. The cross-section of doublets of microtubules looks like a figure eight, since the two microtubules stick together along a line. There are nine doublets forming 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.

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 Paramecium, to move around in search of food, rather than wait until food came to it. Since then, cells as part of larger systems continue to use cilia to great advantage.