Simonc1973 teaches you how to work out the circumference of a circle. You start with a circle and draw a line across the circle. That's called the diameter. You multiply the diameter with Pi, which is 3.141. The video is even more amazing being taught by Harry Chapple, a 4 year old MENSA member, who is a maths child genius. He simply takes the ruler, measures the diameter and then calculates the result of multiplying it by Pi's value. The common value for Pi is 3.14. For very accurate results you can extend the value to more than that.