Science Questions and facts

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I've been itching to post this for a while now, but I wanted to think of a way to share it so it didn't end up in a few post discussion thread, so I thought I'd start somewhere that we can share and discuss science questions and information. Basic idea of the thread, is to ask and answer science questions (homework or otherwise) that you have been wanting to find out about, and share interesting science related facts that you have come across

Anyway, the following is from a literature review that I wrote in November: