When I was a kid, my dad once had me measure the diameter of a bicycle wheel with a tape measure. Then he had me roll the wheel one full turn, mark where it landed on the ground, and measure that distance too. When he asked how many times bigger I thought that distance would be, I guessed "about three times," but measuring it for real showed it was just a little bit more than three times. That little bit extra turned out to be exactly what pi is.
Pi is the circle's circumference divided by its diameter. Remarkably, this value comes out to exactly the same 3.14159..., no matter how big or small the circle is. Whether it's a small coin or a huge circle drawn in the yard, dividing the circumference by the diameter always gives the same number — which sounds a little surprising the first time you hear it. That's exactly why mathematicians of the past spent centuries chasing down its precise value, and today computers have calculated it out to trillions of decimal places.
The best way to learn pi in class is still to roll something yourself. Wrap a string or tape measure around a round object to measure its circumference, then compare it to the diameter, and you'll naturally develop a feel for "oh, it's always a little more than three times" without ever memorizing a formula. That feel turns out to help a lot later on, once you get to formulas for the area or volume of a circle.
Pi doesn't just live in math class. It shows up in designing a roller coaster's circular track, calculating the size of a pizza, even working out how far a spacecraft travels while orbiting the Earth. We usually round it to 3.14, but fields that need very precise calculations sometimes use far more decimal places.
It's also remarkable that pi's digits keep going forever past 3.14 without repeating. A number that never repeats and never ends like this is called an "irrational number," and pi is the classic example. The ancient Greek mathematician Archimedes drew polygons both inside and outside a circle, growing the number of sides all the way up to 96, narrowing down pi's value more and more precisely with each step. Without a calculator or a computer, chasing an answer that doggedly using nothing but hands and shapes always strikes me as remarkable. Students today still just approximate pi as 3.14, but if a kid ever wonders why the digits go on forever, sharing this bit of history can be a great way to spark their curiosity.
One fun fact: March 14th is celebrated as "Pi Day," named after the fact that 3.14 matches the date (3/14). Plenty of schools around the world mark the day by handing out slices of pie while celebrating pi. On our activity page, you can change the diameter yourself and watch an animation of the circle rolling along. Seeing with your own eyes the moment where you go "oh, it really does go a little farther than 3 full turns" is, I think, the fastest shortcut to really understanding this idea. I'd also recommend picking a few round objects around the house — a cup, a plate, a clock — and actually measuring their diameter and circumference with a tape measure, then dividing circumference by diameter yourself. Even with a little measurement error from object to object, once you've confirmed a few times that the result always lands around 3.1, pi will start to feel like a much more familiar number.