If we are to report the results of a measurement to someone who wishes to repro- duce this measurement, a standard must be defined. It would be meaningless if a visitor from another planet were to talk to us about a length of 8 “glitches” if we do not know the meaning of the unit glitch. On the other hand, if someone familiar with our system of measurement reports that a wall is 2 meters high and our unit of length is defined to be 1 meter, we know that the height of the wall is twice our basic length unit. Like- wise, if we are told that a person has a mass of 75 kilograms and our unit of mass is de- fined to be 1 kilogram, then that person is 75 times as massive as our basic unit.1 What- ever is chosen as a standard must be readily accessible and possess some property that can be measured reliably. Measurements taken by different people in different places must yield the same result