I feel that "What If?" is one of the best definitions of Science Fiction. It covers almost every aspect of the genre, which is hard given how diverse it is. However, Fantasy also falls into this definition. The Fantasy/Science Fiction divide is a subtle one, but I think there is an important distinction. Science Fiction is typically more grounded in reality, dealing with things that could have or might still happen (thank you for the cell phone, Star Trek). Science Fiction tends to follow the laws of physics, or comes up with rational reasons for the exceptions (gravity isn't working because we have antigravity fields). I realize that there is a great deal of middle ground, unicorns could be genetically engineered and zombies can be created either with Voodoo or disease, but the general distinction holds true. So, I suppose a more specific definition would be, "Considering the workings of the universe, what if...?"
I have a bit of Science Fiction experience. I was raised on Star Wars, my entire family is a bit obsessed. My 3 year old twin brothers watch atleast 2 or 3 episodes a week (they probably know more about the Star Wars' universe than our own). I've always loved SciFi books, when I was little I was obsessed with L'Engle and K.A. Applegate (Animorphs rocked, Remnants ended depressingly). And The Giver was excellent, one of the best books in the genre for that age group. I'm currently trying to finish all of the Orson Scott Card books (have yet to find a library with a complete series).