Measurements and Standards
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Three questions we often ask:

                    What time is it?

                                            How far is it?

                                                                How heavy is it?

To find out, we need to compare things with standards, so that we can have universal agreement on the answers to these questions. By such agreement we decide to establish these standards as the International System of Units (SI) - better known as the metric system. These standards were set at the 11th General Conference on Weights and Measures in 1960. The agreement set the unit of time to be the second, the unit of length to be the meter, and the unit of mass to be the kilogram. Interestingly, even though other basic units were defined, these three are all that are needed to define every other physical quantity. For example, speed need not have its own basic unit, since speed is just how far you travel (meters) in a particular time interval (seconds). 

Actually, there are four more units that are defined in the SI system. These are the Ampere (used as the basis for making electrical measurements), the Kelvin (used to measure temperature),  the mole (used to measure the amount of substance), and the candela (used to measure luminous intensity).

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