Forestry is applied mathematics part 1: using pi

"What is pi?" If I am asking this aloud, it must be the second teaching week of the university academic year. I am delivering the first lecture in our 'Measuring Trees and Forests' module to the first year National School of Forestry students studying FdSc Forestry, BSc (Hons) Forest Management and BSc (Hons) Woodland Ecology and Conservation at the University of Cumbria. "What is pi?" After a moment of stunned silence (does he really expect us to answer him?), the answers begin: "Approximately three." "Three point something." "Twenty two divided by seven." "Apple." After a pause, while we all consider the awful inevitability of this joke, one of the quieter ones, who has been waiting for their moment says: "Three point one four two." Or even, occasionally, "Three point one four one five nine, erm ... two ... I think." This is followed by appreciative nodded heads and smiles from the other students. I can see them all thinking "That will show him. We, at least one of us, have nailed that answer." "No." I say. "It is none of those things."