frequently reflected on how the roles were working, so that everyone participated actively.)

“Get started, scientists,” Mr. Dolens said. The class went right to work.

Over the next two days, during science time, the class worked on answering the measurement question. Many times they sent a member of their group back to the wall chart to look at the data and take notes. Two groups asked Mr. Dolens for permission to borrow his tape measure, and they remeasured the girls with their shoes on. Another group carefully measured the height of the heels of the three boys’ shoes. There was a good deal of talk about whether to measure in inches or centimeters, something the students had been doing a lot of in math class. Each group ended up deciding to use inches (probably because this was what Mr. Dolens had used). One group noticed that one of the boys was wearing a different pair of shoes and had him stand next to the chart. He was slightly taller now, although not by much.

At last it was time for the “Measurement Congress,” as Mr. Dolens called it. He explained to his students how scientists come together to explain their processes and their findings and take questions from the audience.

Each group arrived at the rug with documents, a poster, or chart paper. One by one, they presented their decision and their reasons. The first group to present included two reporters, Shandra and Coral. Shandra spoke first.

“At first we couldn’t decide, based on the chart. We figured you couldn’t do it both ways—measure some kids with shoes on and some kids with shoes off, because that wouldn’t be fair.”

Both Ms. Martinez and Mr. Dolens knew that if their students were simply told to measure length in a unit such as a centimeter or an inch, they would develop very little understanding of the principles of measurement. Even children who appear to use rulers and scales appropriately often do not understand core ideas like the zero point, iteration, constant units, and tiling, for example. What is important for success in science, in contrast, is having a solid theory of measure that encompasses several kinds of measurement and units. This involves much more than understanding how to measure things.

Over the course of many different measurement activities, Ms. Martinez and Mr. Dolens guided their students in discovering and exploring a number of key principles about measurement, including:

  1. Appropriate units

    Use units of measure appropriate to the thing being measured. Units that work for measuring the length of your driveway may not work for measuring the length of your notebook.

  2. Identical units

    To say that a candy bar is 5 inches long means that every inch is exactly the same.

  3. Measurement conventions

    Standard units like centimeters or inches exist as the result of discussions and agreements among people about measurement problems. Because children will invariably encounter conventions in science, they need opportunities to learn why and how such conventions are established. When children participate in the process of forming conventions, they come to see their utility.

  4. Iteration

    Measurement means repeated applications of identical units.

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