Pi

Pi is a special number in math. If you take any circle — any size, anywhere in the universe — and measure how far it is around the edge, then divide by how far it is straight across, you always get pi. The number is about 3.14, but it goes on much longer than that.

Pi is an "irrational" number. That means it cannot be written as a simple fraction, and its decimal places never end. Mathematicians have used computers to find more than 100 trillion digits of pi, and no pattern ever appears. The digits just keep going.

This is stranger than it sounds. A circle is a simple shape. A little kid can draw one with a string and a pencil. But the number hiding inside every circle is endless and complicated. No matter how carefully you measure, the answer always escapes into more decimal places.

Pi shows up far beyond circles. It appears in equations about waves, vibrations, chance, the orbits of planets, and even the shape of DNA. The ancient Greek thinker Archimedes, around 250 BCE, was the first person to carefully calculate pi. The symbol π (the Greek letter pi) was first used in 1706 by a Welsh mathematician named William Jones. Because pi starts with 3.14, people celebrate Pi Day each year on March 14.