Today, millions of people worldwide will celebrate Pi Day... the mathematical number.

Pi is the ratio used to compute the circumference, area, and volume of circles, and it works out to3.141592653 for the first ten digits.

It's written using the Greek letter π, pronounced "pie."

Schools often mark the special day by taking part in Pi recitation contests, with the winner getting a pie as the prize.

Tech and science companies also love Pi Day with Raytheon Company employees delivering apple pies to middle and high school math teachers within 3.14 miles of company offices in Arizona, California, Florida, Maryland, Massachusetts and Virginia.

Pi Day gets extra credit because it's also Albert Einstein's birthday, who was born in 1879, and it was first celebrated in 1988 at San Francisco's Exploratorium science museum.