Happy Pi Day! Every March 14th (3/14 using the day/month format used in the U.S. to write dates) mathematicians and students around the world celebrate pi, which is a very special number. Pi (the Greek letter “π”) is the symbol we use in math to represent the ratio of the circumference of a circle to its diameter, which is equal to approximately 3.14. That’s why we celebrate pi day on March 22—the date 3/14 has the same digits as the first three digits of pi! But the number pi actually goes on for quite a while. It’s more like 3.14159265358979… the numbers after the decimal point continue forever, without any pattern. In fact, pi has been calculated to over one trillion digits beyond the decimal point. I’m glad I don’t need to memorize that for an exam!

The first large-scale pi day celebration took place in 1988 in San Francisco, my hometown. It was organized by physicist Larry Shaw, who was working at a science education museum called the Exploratorium. The staff of the museum, as well as visitors from the public, celebrated by walking around in circles and eating fruit pies—because pies are round, like circles, and because in English “pie” and “pi” are homophones. That means the words, when spoken, sound exactly the same.

Today, pi day is still celebrated by eating pie, reciting as many digits of the number pi as you can remember, and solving math problems that involve the constant pi.

And one additional fun fact: Albert Einstein’s birthday is also 3/14. So in some places, people also hold contests to see who can dress up to look most like the famous physicist!