# Pi for Kids | What is Pi | Pi Day March 14th | 22/7

Pi is a significant symbol in math and physics. It is used to understand anything that is circular, and it can even help you figure out how much pizza you have. Pi is an interesting number that never ends. Most of the time, we shorten pi to a few digits, but some people make it a game to memorize as many digits as they can. In this video, our puppet friend Charlie celebrates Pi Day by learning about the definition of pi. Pi is the ratio of a circle's circumference to its diameter. It is also in the equation for the area of a circle. The area of a circle is pi times the radius squared. Charlie uses this formula to figure out what is a better deal: one 18-inch pizza or two 12-inch pizzas. Do you celebrate Pi Day? How many digits of pi do you know by heart?