Cut the yellow yarn into various lengths, between one and four feet long.
Tie each length of yarn to itself, creating a circle.
Cut the blue yarn into two foot long lengths, one piece for each circle.
Provide each student with a yellow circle, a blue length of yarn and a pair of safety scissors.
Instruct each student to lay out their yellow yarn into a circle at their work space.
Ask each student to lay their blue yarn across the circle.
Help each student cut their blue yarn where it crosses the edge of their yellow circle.
Show each student how to cut their yellow circle and lay it out next to their blue yarn.
Collect the students' lengths of yarn.
Hang the lengths of yellow yarn on the bulletin board, from shortest to longest.
Hang the lengths of blue yarn on the bulletin board, from shortest to longest.
Discuss with the students whether or not the yellow lengths - the circumferences of the circle - and the blue lengths - the diameters - seem to have any relationship. That is, as the yellow lengths get longer and the circles get bigger, do the blue lengths get longer too?
Explain that the relationship between the two, the circumference and the diameter, is a fraction: C/D = pi, and that this is true for every circle, no matter how big or small.