"My students love to grow plants, they are excellent caregivers when motivated, and they also love contests," reports Nashville, TN, teacher Nancy Johnson. "I combined these facts with the need to teach my fourth graders the metric units of height, weight, and volume by setting up a contest to see which small group could grow the 'biggest' plants." Nancy provided the lima bean seeds and soil, and students discussed how to keep the race fair. They realized that they'd have to keep all growing conditions the same, Nancy reports. After all, if Johnny's group used fertilizer and other groups didn't, that wouldn't be fair, they reasoned. "My main focus," reports Nancy, "was on giving students an incentive to hone their metric measurement skills. Beyond that, I wanted them to think about controlling variables to provide a standard set of conditions."
To decide how to determine "the biggest" plant, students used texts for information about metric measurements for height, weight, and volume. They first used rulers to measure heights of standard objects to improve measurement skills, then measured the heights of their groups' plants. "Students wanted to measure the heights of the other groups' plants too," says Nancy, "to be sure that the 'competition' was also measuring correctly!" To reduce individual competition and increase cooperation, they totaled the sizes of each member's largest plant. Each child made an individual data chart for his or her journal, and the class made a group data chart to track the whole contest.
Students next learned how to find the mass of standard objects, then applied this to finding the mass of their own plants. "We had a lively discussion of the sources of error in finding the mass of plants!" says Nancy. "No one wanted to compare the mass of their plants, since it seemed that we'd have to take the plants out of the soil and dry them first." After learning how to use water displacement to find volume, students considered how they might find the volume of plants. But after much discussion, most students agreed that height in centimeters was the most accurate and fair measurement of "biggest" in this contest!
Nancy characterizes the classroom during the contest as very active, with students working in groups of three using the measuring tool of the day, discussing results, and recording them on the classroom wall chart. "This hands-on, minds-on investigation helped students develop meaningful concepts about metric measuring. They learned about factors that influence plant growth, and became more autonomous as they set up the rules and conditions, grew their plants, and developed confidence in their ability to learn, to decide, and to control their world." She adds that once students had a good idea of keeping careful records and controlling conditions, the next step was to discuss how they might test the effect of one variable -- amount of light, for instance -- on plant growth.