The myriad organisms that inhabit our earth, including us, are in the process of change, of a sort, all of the time. Some of that change is due to aging, some to growth, and some to reproduction. It is an interest of scientists to measure these changes to note patterns, trends, and anomalies. These measurements inform decisions about global warming, drug effectiveness, plant resistance, and Mars landings. In order to measure and document change, tools are necessary. These tools are concrete objects, like rulers, maps, and thermometers, and three-dimensional models. They are also electronic, like sensors, simulations, and telescopes. And, they are mathematical, like counting, calculus, and algebraic models.

Each tool is useful for its task, but there are limitations to its accuracy. Every tool measurement is subject to a certain amount of error. The accuracy and error of tools is an important consideration in the measurement of growth and change. Also, sometimes tools must be devised to measure some thing or phenomenon more accurately or to measure at all. Then, the tool must be reliable and meet all the essential requirements of measurement. To participate in science, all of these pieces of measure must be constructed and applied.