A circle is formed by moving around a fixed point, while staying the same distance from it. Compasses allow us to draw in circles - as do a pencil and a length of string. The distance across the circle, and the distance round it, are related.
If you wrap a string around any circular object and then lay it across, you will find that the circumference is roughly three times the diameter. The circumference of any circle is a fixed number times the diameter. This fixed number cannot be written as a fraction or a decimal, so the Greek letter pi(9) stands for it - about 3.14. There is another way of calculating 9. You need a boarded floor and astick the same length as the boards are wide. Drop the stick many times, and count how many times it falls on a crack.
Twice the number of times you dropped the stick, divided by the number of times it fell on a crack will give you a value for 9. For example, if you dropped it 100 times and it fell on a crack 62 times your value for 9 would be 20062 - or 3.2. The more you drop the stick, the more accurate the figure.This is because the stick turns as it falls.
Whether or not it crosses a crack depends upon where the centre falls, and how it turns as it falls. When a stick turns around its centre, it is moving in a circle.