Which stack is the best fit for "childish", naive questions of general interest? Sorta of questions children like to ask. For example - "How does it comes that water in oceans is salt, and in rivers it isn't".
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Stack Exchange sites are organized around topics, not difficulty levels. In actuality, all of the sites make a serious effort to appeal to experts, so that there are people who can provide good answers to all the questions coming in. Without the experts, you'd end up with a bunch of ill-informed passers-by tossing out speculation. Of course, non-experts are welcome on Stack Exchange sites so long as they can compose coherent, on-topic questions.
Each site deals with a specific subject; there will never be one that says "come ask whatever pops into your head". This prevents (or reduces) questions getting buried under millions of other questions you don't care about. When the site has a specific focus, it's easier to build a community of experts around that subject. It's also because open-ended question sites have a history of doing poorly.
Your example question though might fit on the Popular Natural Science site if it were to launch.
For other questions, consider this excerpt from What kind of questions should I not ask here? which applies to pretty much all Stack Exchange sites:
If your other questions meet those guidelines, you should be able to find or propose a Stack Exchange site where you can ask them.
I think this question is essentially the same as this: Could we create a place for the unwanted 'General Reference' questions from the Stack Exchange family?
To copy Dori's answer:
This may also be relevant.