At Z's school, they were taught how to measure the circumference of cylinders. They do this by marking a point on the circumference and then rolling the cylinder along a straight line until the marked point touches down again. Then, they take the flat side and measure the diameters off against the same line. They've done this for a whole lot of circles. Every time they do that, they see that it takes about 3 diameter-lengths to cover the length of the circumference. Now, they're dividing the circumference by the diameter and finding that all the numbers are approximately 3.14. Finally, they had a lesson in the meaning of PI.