Fantasy vs. Science Fiction

I haven't posted a "musing" in a while, so I thought I would today. Something I've always wondered is what specifically makes something "science fiction" and makes something else "fantasy". The genres are often combined, probably because they're both so rooted in imagination. Science fiction is often somewhat realistic, but not always. Often the "science" of the story wouldn't even come close to working, but it sounds smart enough that the readers/viewers buy into it. Fantasy, on the other hand, doesn't even bother seeming realistic. That would rather defeat the point. In my mind, the purpose of fantasy is to explore impossible ideas  and stretch imagination to the limit. Science fiction retains a tenuous hold on reality, while fantasy is pure make-believe. But both clearly have their place in fan's hearts, and both can be legitimate literature. I believe that despite their imaginary elements, fantasy and science fiction can be used quite effectively to inspire people and even cause change. Science fiction, especially science fiction based in a future twisted by current events, can be a very good tool for social criticism. This kind of science fiction is the one usually accepted as literature, and it can be very powerful. I still think of certain science fiction short stories I read in middle school when contemplating or discussing certain political or environmental issues. Science fiction, therefore, has a clear role in literature. Fantasy, however, is often dismissed as mere escapism. I don't believe that, however. Fantasy's role in literature may not be as clearly defined, but it is there nonetheless. Even the most unbelievable fantasy stories have subtle messages that matter in the real world. And sometimes the unrealistic nature of fantasy allows these messages to have a more profound impact on the reader simply because they are more immersed in the story (see my post "The Power of Fantasy" for further discussion of this topic). Science fiction often critiques and reflects the outer elements of our society, while fantasy does so much more for the inner lives of individuals. Either way, both are a force to be reckoned with.