March 14, you may be surprised to learn, is Pi Day. Why? It’s 3-14, which is pretty close to 3.14, pi.

If you don’t remember much from geometry class, pi, represented for centuries by the Greek letter “π” is the ratio of the circumference of any circle to the diameter of that circle. It is the most recognized mathematical constant in history.

It’s first three digits are 3.14, but it is actually infinite.

Schools have fun with Pi Day, which is celebrated unofficially on March 14, which is also Albert Einstein’s birthday (he was born on that day in 1879.) Kids dress up as Einstein, people make pies and decorate them with the symbol for pi.

And the Microsoft store is offering 3.14 percent off of some laptops!

Here’s a refresher video on pi: