Pi is a constant (never changes) because it is a ratio. It is the ratio of the circle's circumference and its diameter. The larger the circumference, the larger its diameter. So Pi will always be the same. A ratio is basically a division of two numbers. To make it simple, think of a circle as having a circumference of 10 and a diameter of 5 (Remember, this is not true, but I am making it very simple). The ratio would be 10/5 which is 2. A circle of 20 circumference would then have a diameter of 10. 20/10 is still 2. So, now change the value to real circle and divide the circumference by the radius (like I did), and the answer would be 3.14, which is Pi. It is always going to be 3.14. By the way, Pi also equals the Area ratio to the radius squared.
