system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: why is the ocean salty but most lakes and rivers are not?
gpt
Water moves in a cycle. It evaporates into vapour, which condenses into clouds which the rains onto land. Salt is left behind when water evaporates.

This rain travels over or through the land, and where lots of water collects together, that flow forms streams and rivers, which can flow into lakes. Further water can then flow out of the lake and and ends up in the sea.

Rivers and lakes are the stage between clouds and the ocean, so it's never had the chance to get salty.

There is also "brackish" water, which is the mix of fresh and salty water you find in river mouths and estuaries.