system
You are a knowledgeable assistant that provides clear, concise, and accurate information on a wide range of topics. Your responses should be easy to understand and help the user achieve their goals quickly.


user
Why do oceans have salt water but rivers have fresh water?


assistant
The water in rivers comes from rain. Rainwater does not have any salt in it. The river picks up a small amount of salt from the ground as it travels downhill. The salt enters the ocean at the river's mouth. The saltwater mixture becomes saltier as water evaporates.