system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we just base the weight of a kilogram by saying that it is the weight of 1L of water instead of using the International Prototype Kilogram?
gpt
1L of water at what temperature and pressure? A liter is a unit of volume, and a liter of steam is going to have a lot less mass than a liter of water at the bottom of the Mariana trench. Most human life exists in a similar band of temperature and pressure, so 1L of water weighs about a kilogram, but it lacks the precision of a set prototype kilogram.