Archimedes' constant. It's the ratio of a pie's circumference to its diameter. Very useful for determining how much pie you have: Simply multiply the square of your pie's radius by Archimedes' constant (pi) to find the surface area. Multiply that by the thickness to find the volume! Note: your pie pan is probably not a cylinder. For precision, be careful to measure the radius on a plane half way up the pie's thickness.
Impress your Grandma next holiday!