system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why is the ocean made of salt water and the lakes made of fresh water?

Are really big lakes saltier than small lakes? Does the salt in the ocean change around, or is it always the same salt?
gpt
Oceans initially eroded away at bedrock and sediment that put salt into the water. Lakes and rivers, however, receive their water from rainfall. When the sun heats up the ocean, water evaporates and leaves the salt behind. In gas form, the water travels back to the mainland where it cools off, becomes rain and supplies the lake with salt-free water. That water then travels back through the rivers, into the ocean and becomes salinated again.