Today, if you head over to Google’s homepage, you’ll notice something a little different — the logo has been swapped out for a colorful Pi Day Doodle, and it’s honestly a pretty cool one.​

March 14th is Pi Day, and the date itself is the whole reason for the celebration — 3/14 matches the first three digits of the mathematical constant π (3.14…). It’s one of those nerdy holidays that somehow managed to go completely mainstream, and Google’s been enthusiastically marking it for years.

This year’s Doodle pays tribute to the ancient Greek mathematician Archimedes, who came up with one of the earliest and most clever ways to estimate pi. His trick? He sandwiched a circle between two 96-sided polygons — one inside the circle, one outside — and used the perimeters of those shapes to pin down upper and lower bounds for pi. No calculators, no computers, just pure geometric ingenuity. This method, known as the “method of exhaustion,” was so effective that it remained the primary technique for calculating pi for nearly 1,900 years.

Pi Day was officially born in 1988, when physicist Larry Shaw organized the first celebration at the Exploratorium science museum in San Francisco. It was delightfully simple — Shaw and his colleagues walked in circles and ate fruit pie. The holiday got its official U.S. recognition in 2009 when the House of Representatives passed a resolution designating March 14 as National Pi Day. And yes, it also happens to be Albert Einstein’s birthday.

Today, the day is recognized globally by UNESCO as the International Day of Mathematics, and this year’s theme is “Mathematics and Hope” — a nod to how math connects people and drives innovation across the world. Celebrations typically include pi-reciting contests, math challenges in schools, and of course, plenty of pie.

Pi itself is an irrational number, meaning its decimal digits go on forever without any repeating pattern — currently calculated to over 100 trillion digits, though Archimedes managed to nail it down to between 3.1408 and 3.1429 with just a polygon and some patience.