system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can water filters remove 99.9% of impurities but not turn salt water to fresh water?
gpt
The impurities referred to are larger particles and very large molecules. Unfortunately for us in this case, salt is an extremely tiny molecule -- not much bigger than a water molecule, and far smaller than normal filters. Only a very special filter, requiring very high water pressure, can filter out salt molecules.