Science is a process for acquiring knowledge that depends on making careful observations of objects and events in our universe and on forming theories that explain these observations. Scientists rely on their own senses, instruments that enhance those senses, and instruments capable of detecting events or occurrences that humans cannot sense at all -- such as magnetic fields.

Various tools have been developed over time that enable us to study visible objects as well as objects too small or too distant to be seen with our eyes. Using modern telescopes, astronomers can see distant portions of the universe and explore its origins and its behavior. Using microscopes -- now aided by computers to construct complex images -- scientists can see the structure of molecules in fine detail and understand how they function in living systems.

Measuring the things we observe allows us to compare size, shape, distance, speed, and other physical and chemical properties. To measure size and distance, for example, scientists and engineers use the fundamental property length, which is the measurement of space in any direction. The earliest measurements of length were made using familiar objects. One foot, for example, was originally the length of a man's foot from the heel to the tip of the big toe. Many instruments, such as thermometers and rulers, display scales for making measurements.

Celsius and Fahrenheit are examples of different scales that measure the same thing, in this case temperature. For many years, scientists and engineers sought a standard measurement system so that measurements could be understood by everyone without needing to convert the units from one scale to another. The metric system is now the standard measurement system for most fields of science. It is not based on human anatomy; rather, relationships between its units -- such as grams, meters, and liters -- are based on the number 10. Thus, calculations utilize the decimal system, making these units much easier to work with than, for example, 12 inches in a foot and three feet in a yard.