In this activity, students build a trundle wheel and use it to measure distances in the classroom. The trundle wheels need to be stored in the classroom for use in the next lesson. If it is not feasible to store a trundle wheel from every group of 3–4 students, combine them to form larger groups before building the wheels. Each student should still get a chance to practice measuring with the trundle wheel.

It is suggested that students build their trundle wheels using a paper plate as the wheel, two rulers taped together end to end as the handle, a metal paper fastener, and an index card taped to the wheel to produce an audible “click” when it hits the handle. There are many other ways to build a trundle wheel. If this fits into the culture of the class, students can use other designs and materials, for example, students could use a bike with a playing card in the spokes to count rotations.

The goal of this discussion is for students to double-check that their trundle wheels work correctly and their results are reasonable. Invite one group that used each size of paper plate to demonstrate how their wheel works by walking across the classroom and counting the “clicks.” Record the data for all to see and discuss how these groups’ calculations for the length of the classroom compare, which may include the following points:

• The group with the smaller plate had a larger number of clicks, and vice versa.
• Each group needs to multiply their number of clicks by the circumference of their wheel.
• The circumference of the wheel can be found by multiplying the diameter times or the radius times .

If these ideas do not come up during the discussion, consider asking:

• "Should the answer be reported in terms of or should an approximation be used?"
• "Is it possible to count half rotations of the wheel?"
• "What is the same and what is different about how groups used their trundle wheels?"