system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How did the first weight get weight to know how much it weights?
gpt
It more-or-less went, "We need to define a weight, so we take this object and say that it defines the unit and calibrate our scales to match." In the case of the kilogram, it was defined as "the mass of one liter of water," and so no matter how massive one kilogram of water actually was, that mass set the definition.