What is a kilogram?

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Vintage iron weights on the white background.

We think we know what a kilogram is, but do we really? While it seems to be a simple question, the definition of a kilogram has changed multiple times since its invention in the late 1700s. The kilogram is a base unit of mass in the metric system. Therefore, there must be no confusion as to what a kilogram is. Everyone must be able to agree on its precise mass, especially scientists and engineers when making important calculations. In the late 1700s, one kilogram was defined by the mass of one litre of water, but this approach had a practical problem.