system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the difference between forward and reverse osmosis?

I see "reverse osmosis water" on a lot of products these days and I'm curious how reverse osmosis is different.
gpt
In osmosis, water travels through a membrane from a low-concentration (i.e. less dissolved solids) mixture to a high-concentration mixture until the concentration on both sides is the same.

This happens spontaneously and can be quite dramatic if one side is extremely highly concentrated.  This is why drinking salt water is so harmful, osmotic pressure starts sucking water out of your cells.

*Reverse* osmosis is the inverse process, forcing water to cross from the concentrated side to the pure side.  This *won't* happen naturally and requires high pressures, but also acts as a very effective filtration process because the membrane excludes most particulates and large ions.