I Have No Capacity for This!
by John Pearson
Last week, my kids and I explored the mathematical concept of capacity. This was a new experience for all of us, as the kids were unfamiliar with the topic, and I had never taught it before. Capacity, or the amount of liquid that a container can hold, is a brand new objective for third grade this year.
It's also a topic that I was pretty unfamiliar with myself. As a former engineer, I knew a lot about measurement in terms of length and temperature, but I don't have much experience with pints and liters. There were a few questions I needed to answer before trying to teach it.
"How many cups are in a quart?”
"Why does the abbreviation for fluid ounces have a ‘z’ in it?"
"Is it ever truly acceptable to use the word ‘dram’ in civilized conversation?"
Monday morning came with the anticipated confusion. For kids who have trouble remembering the difference between a square and a rectangle, capacity seemed doomed from the quart.
Going over unit conversions seems pretty pointless, since most of the units were words the kids had never even heard before. I may as well tell them 2 snerks equals 1 plekt, and 8 crells in a doogy.
Instead, we focused on real-world examples. I showed them a milk jug to represent a gallon, and a Gatorade bottle to represent a quart. A peek into my home fridge led to the discovery that a bottle of Kraft ranch salad dressing can hold exactly 1 pint, and so I brought that in for show and tell as well. I found something that has a capacity of exactly 1 liter, but I opted to leave the bottle of Absolut at home.
Roughly 40 containers with a 1-cup capacity are delivered to my room every morning. The milk and juice cartons that are part of the well-balanced breakfast served as helpful physical examples. Printed right on the side of each cartoon is, "1/2 pt." Strangely, this made me think of Little House on the Prairie, which in turn made me feel very old.
Inspired by my own kitchen raid, I gave the kids an extra credit homework assignment. I asked them to go home and look through their refrigerators, freezers, and pantries. They made lists of all of the containers they found, along with the capacity of each.
There were a few examples of missing decimal points – 612 gallons of barbecue sauce, 277 gallons of mustard, 118 liters of Mr. Clean. Either that, or someone owns stock in the wholesale store.
The kids didn't limit themselves to food and drink alone. Lists included body oil (473 mL), Dawn dish soap (1.18 qt), and Tilex (1 pt). Soda and milk were on most lists, but items like Ms. Butterworth (1 pt, 8 oz), champagne (1.05 pt), and Hooter’s hot sauce (5 fl oz) were unique.
Perhaps with all of the varying liquids, it was inevitable that Maalox (355 mL) would make an appearance as well.
By the end of the week, the kids seemed to have a decent handle on the concept. Only time will tell, of course, but I feel confident that my kids will, at the very least, never walk into a store and ask for 10 pounds of beer.
John Pearson is a third-grade math and science teacher in Dallas, Texas. He has degrees in mechanical engineering from Duke University and Texas A&M, so most consider his math abilities adequate enough to teach nine-year olds. He is also the author of Learn Me Good (Lulu, 2006), a funny, fictionalized account of his first year in education. Read more at www.learnmegood.com