When teaching parts of a circle — pi, area and circumference — I use a tool we all have readily available: the floor! I use washable overhead markers and draw a large circle, covering about 1/3 of my floor. I start by taping a piece of string to the center of the floor and tying the marker to the other end, which works as a compass to make a perfect circle.
I draw the diameter, a radius and a chord. My students “walk” each part as we define them. To understand pi, the students walk the diameter and the circumference of the circle. Then we count their steps. Once we display the data on a chart, it’s easy to see that the walk around the circle took about three times as many steps as the walk across the circle. I find that kids don’t forget what pi means, since they literally walked it!