Try rolling the circle with different diameters. Watch the tick marks and see for yourself that the length the circle travels in one turn (the circumference) is always about 3.14 times the diameter. That value, 3.14…, is pi (π).
Pi is a circle's circumference divided by its diameter. Amazingly, this value always comes out to 3.14159…, no matter how big or small the circle is. Whether it's a coin or a giant circle drawn on a playground, dividing the circumference by the diameter always gives the same number.
Pi keeps going forever past 3.14 without ever repeating — a number like this is called irrational. The ancient Greek mathematician Archimedes, without a calculator or a computer, narrowed down the value of pi more and more precisely by drawing polygons inside and outside a circle and increasing the number of sides.
Change the diameter with the slider and roll the circle. Watch the tick marks and see for yourself how many "times" the diameter — not how many full turns — the circle travels.