Introduce your math class to the perimeter of a circle. Teach them that pi describes the relationship between the circumference and the diameter. As a hands-on investigation, pairs or small groups use string to compare these two values on different circles that you provide (Consider a cookie, pizza, paper plate, etc.). They record their data in a chart and then calculate the ratios of c to d. Finally, they graph the data and discover the relationships! This is an outstanding lesson that brings ratio and proportions around!