TED-Ed: The infinite life of pi - Reynaldo Lopes

The ratio of a circle's circumference to its diameter is always the same: 3.14159 and on and on (literally!) forever. This irrational number, pi, has an infinite number of digits, so we'll never figure out its exact value no matter how close we seem to get. Reynaldo Lopes explains pi's vast applications to the study of music, financial models, and even the density of the universe.