Taking several sized lids, I asked the kiddos to trace five different lids on a piece of plain legal-sized paper.  Using a centimeter tape measure, I asked them to measure the circumference and diameter of each lid and jot that down on their papers beside the appropriate drawing.

Using their measurements, I held my fingers on the tape measure to show the circumference and diameter of each lid.  I asked them to think about what they noticed that was similar about each of the measurements.   (The diameter is always about 1/3 of the circumference.  You can show this by folding the tape measure in thirds each time.)

Once they saw this “almost 1/3″ measurement concretely, I told them there was a way to figure out the circumference accurately every single time.  All they need to know is the diameter of the circle.  I secretly held a calculator and asked them to give me the diameter of several of their lids.  I would multiply the diameter by Pi (3.1416) and get their circumference measurement every time.

This, piqued their interest of course, and they couldn’t wait to find out the magic trick.  So, I showed them the formula C = (Pi)d (I don’t know how to make the Pi sign on the computer!) and allowed them to use the calculator to find several circumferences around the house.

They were so excited to tell their Dad that they will be able to help him decide what size tire he needs to buy the next time a giant tractor tire needs to be bought.