The metric system (SI) defines seven fundamental quantities that cannot be further broken down, from which all other derived quantities come. The meter is the fundamental quantity for length. Area uses the derived quantity of square meters and volume uses cubic meters.
When measuring very small or very large units, use of the fundamental unit often becomes cumbersome. Measuring the width of a human hair using the meter requires a decimal point and several zeros. The SI system handles this prefixes that describe multiplying the base unit by a power of ten. Measuring the hair in nanometers (0.000000001 meter) is more appropriate.
Units such as the day, hour and astronomical unit, are not officially a part of the metric system, but they are often used in science. These units are also possible to derive from the SI fundamental quantities. For example, a day is 86,400 seconds, according to HowStuffWorks.
Some derived units have special names. For example, the metric system also uses the liter to measure volume. One liter is 1000 cubic centimeters. The Newton, for measuring force, is the product of acceleration (meters per second squared) and mass (kilogram) or kilogram meter per second squared. Many of the special names given to the derived units honor the work of scientists in the area the unit represents.