An ellipse is an oval shape, like a circle that's been gently stretched or squashed in one direction. If you trace around the edge of most dinner plates, you draw a circle, but if you tilt that plate and trace its shadow on the table, you've drawn an ellipse.

Ellipses appear everywhere in nature and science. The planets don't orbit the sun in perfect circles; they travel in elliptical paths, meaning their orbits are ellipse-shaped. This discovery by astronomer Johannes Kepler in the early 1600s revolutionized our understanding of how the solar system works. When you look at Saturn's rings from an angle, they appear elliptical rather than circular. Even the opening of the human eye can look elliptical.

Mathematicians have a precise way to describe ellipses: every ellipse has two special points inside it called foci (FOH-sigh). If you put a pin at each focus, loop a string around both pins, and trace with a pencil while keeping the string tight, you'll draw a perfect ellipse. The closer together the foci are, the more the ellipse resembles a circle. The farther apart they are, the more stretched and elongated it becomes.