That has to be one of the most common question asked of any writer.  After a time, it becomes annoying.  I always try to reply courteously, but I know of several writers who have created standard 'stupid question' responses to that one.

The truth is that story ideas randomly and spontaneously generate in a writers' brain just as flies marvelously hatch from rotten meat.  (Oh. They disproved that?  Well.  Just as flies were once thought to spontaneously generate from rotten meat.)  The problem ...