<|im_start|>user
Why do oceans have salt water but rivers have fresh water?<|im_end|>
<|im_start|>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.<|im_end|>
