Where does a story idea start?
Real life answer: I might, for example, lament to my mother that I wish I were writing a short story. She would then offer up whatever ideas came most readily to mind in an effort to jumpstart my creative process. Some (read: most) of those ideas will be ridiculous beyond comprehension. Others will sound almost useable. And there are yet others that never make it out of her mouth, denied passage by her less than golden short-term memory.
This may prompt me to mention having read somewhere that people are more likely to forget a thing after having just passed through a doorway, which will in turn prompt Mother Dear to ask, “Why could that be?” Obviously, we’ll decide, it’s because some invisible creature lurking in the threshold is snatching our thoughts as we go by. (The only logical explanation, really.) And from that “orderly succession of related events”, I’ll have my story.