A shoeseller meets a mathematician and complains that he does not know what size shoes to buy.  “No problem,” says the mathematician, “there is a simple equation for that,” and he shows him the Gaussian normal distribution.  The shoeseller stares some time at het equation and asks, “What is that symbol?” “That is the Greek letter pi.”  “What is pi?” “That is the ratio between the circumference and the diameter of a circle.”  Upon this the shoeseller cries out: “What does a circle have to do with shoes?!”