You will be asked them, too, so start practicing your answers now. This is every author's most frequently asked question, and we all hate it.
Ideas are the easiest things in the world to come by.
Maybe you do something with it, maybe you don't. Otto from Otherwhere sparked when my husband mentioned that the fog on Woodlawn Lake one January morning looked thick enough to walk across into another world. Stories are everywhere, and if you only keep your eyes open, you'll see them. On the other hand, if the kid's considering a career in literature, this is a legitimate thing to want to know. I answer, not with specifics, but with the theory.