You have probably heard about pi before. Maybe your teacher was talking about it. Maybe you saw it in your math book. Maybe you saw it on television or in a movie. You have heard of it, but do you really know what pi is?

Pi is usually something you will hear about when someone is talking about math. Even though pi looks like a sign, it is really a number. Why not just write the number? There is a reason that we do not write numbers when we are talking about pi. Pi is a number that has no end. It keeps going and going and going! When people need to use pi to solve a math problem, they usually just write pi as 3.14. That is a close guess of the value of pi. If people want to get even closer to the real value of pi, they might use 3.14159. It is close to the real value of pi, but there are even more numbers that make up pi. Some people have spent their whole lives trying to find the true value of pi. Now that we have computers, we can use them to get even closer to the true value. In 2002, a group in Japan used a computer to calculate pi to more than one trillion decimal places!

That may be interesting, but it still doesn't tell you what pi really means. Why would anyone want to use a number that doesn't end? The reason has to do with circles. Pi is a number that is important in understanding circles. Pi is important in finding the circumference, or distance around the outside, of a circle. Pi is important in finding the area of a circle. Area is the measurement of the space inside a two dimensional, or flat, object. Pi is also important in finding the volume of circular objects. Volume is the measurement of the space inside a three dimensional object. Spheres, like balls, and cylinders, like soup cans, are both three dimensional objects that contain circles.