It's Thursday, and time for another retro science video! This time around, a brief (three-minute) explanation of Pi, that mischievous, irrational number. The following video explains visually how to calculate *pi*, and thus where formulae like *pi* * r-squared come from. (I can still remember my geometry teacher making the corny joke for the thousandth time: "Pie aren't square. It are round!").