When I was in 6th grade (U.S. so 12-13 years old), I took a summer school class. The teacher gave us all different sized spools (spools that hold sewing thread but were empty). We each made a mark on one wheel then rolled it one revolution over paper and marked how far it went (i.e. one circumference). Then we held the spool on its side and measured how may diameters it took to cover that distance. And, no matter what size the spool, it took a little over 3 diameters.
In looking back this was a great exercise to prove that circumference equals $\pi$ * diameter. I was wondering if this demonstration is common.