All seven base units in the metric system are based on constants of nature. This means that they can never be destroyed. The seven SI base units are the metre, kilogram, second, ampere, kelvin, mole and candela. All other SI units are derived from these base units. Hence, they are called SI derived units. Anyone with the right laboratory equipment and expertise can reproduce any of these base units. The problem with tying units to physical objects is that they can be destroyed, and they tend to degrade over time.
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