This terrific idea comes from MathWise! Over 100 Hands-On Activities that Promote Real Math Understanding, by James L. Overholt and Laurie Kincheloe. This is one of my favorite resources for great math activities.
First, we gathered our supplies:
Then we talked about different ways to measure our watermelon. First, we put it on a sheet of paper, and used a straight edge to make a mark at each end. This gave us the measurement of the straight line from the stem to the end.
But...how do you measure something that curves? Well, both kids suggested using the tape measure from the sewing kit, but I wanted it to be a bit more challenging! We tried using the metal tape measure, but of course, it didn't conform exactly to the watermelon's shape. It did give us a pretty good estimate.
Then we wrapped a piece of yarn over the melon, cut it at the stem, and measured the yarn. Ta-da! An exact measurement! We did the same thing for the circumference of the watermelon.
We compared the pieces of yarn, and noted the difference in the lengths.
Then we took turns holding the watermelon and estimating its weight. We also picked up a five-pound bag of sugar and a one-pound package of butter for comparison.
Once we had estimated the weight, we wanted an accurate measurement. But our little kitchen scale can't handle something as heavy as a watermelon. And if we put the melon on the bathroom scale, we can't see the dial. What to do?
First, we weighed Super on the bathroom scale. Then, we weighed Super holding the watermelon. Then we solved for the unknown. If Super's weight is known, and Super plus the watermelon is known, then how can we find the unknown? Subtraction!
Once we cut the watermelon, we were able to find the radius and diameter. (We put a bit of plastic wrap over it first.)
Have you ever wondered how much edible watermelon there is compared to rind? We did. How could we figure it out?
Well, we each ate some watermelon and saved the rind. We found the weight of this bowl.
And then we put our rinds in the bowl. We found the weight of the rind with the bowl and then subtracted the weight of the bowl. As it happens, we ate exactly one-fourth of the watermelon. So, we were able to multiply the weight of this rind to get the total estimated weight of the entire rind.
The book also suggests doing activities with the watermelon seeds, but I could not find a seeded watermelon anywhere! That's OK, we'll be doing some pumpkin math soon, and I'm sure we'll find plenty of seeds then!