system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come rivers aren’t salty, but when they get into the ocean it is salty? Where did the salt originally come from if the rivers never had it?
gpt
* The rivers come from freshwater from rain.

* As they flow the collect very small amounts of salt from the earth.

* They deposit that tiny amount of salt into the ocean.....over and over and over.

* When the sun evaporates the water out of the ocean, the salt stays behind. 

* So the rivers are always replenished with fresh water from the air, but the oceans always keep the salt that has been brought to them by the rivers.