How are do we keep units of measurement the exact same over time?

Question

Something I’ve always been curious about – how do we keep units of measurement like centimeters, kilograms, seconds, etc the exact same over time? If we use measurement tools to create other measuring tools over the span of centuries, how do we prevent slight deviations from the original measurements in the long run? For example, when we measure a kilogram today, is that still the precise mass of the original kilogram?

In:

Answer 1

Because we aren’t using tools, in terms of volumes necessarily. One gram is one cubic centimeter of water. We can build from there Bonus fact: One calorie is the amount of energy it takes to raise one cubic centimeter of water by 1 degree.