In Stephen King’s wonderful short book "On Writing: A Memoir of The Craft"––in this writer’s opinion the most accessible how-to book on creative writing––he describes exactly how as a struggling writer he came up with the idea for "Carrie."
“One day while I was working at the laundry, I started seeing the opening scene of a story: girls showering in a locker room…and this one girl starts to have her period. Only she doesn’t know what it is, and the other girls––grossed out, horrified, amused––start pelting her with sanitary napkins. The girls begin to scream. All that blood! She thinks she’s dying and the other girls are making fun of her even while she’s bleeding to death…she reacts…fights back…but how?”
“I’d read an article in Life magazine some years before, suggesting that at least some reported poltergeist activity might actually be telekinetic phenomena. There was some evidence to suggest that young people might have such powers, the article said, especially in girls in early adolescence, right around the time of their first––Pow! Two unrelated ideas, adolescent cruelty and telekinesis came together, and I had an idea.
The next night when I came home my wife Tabby had the pages in her hands. “You’ve got something here,” she said. “I really think you do.”
It was his first blockbuster.
All knowledge is connected to all other knowledge; King's genius is in making the connection. He knows that new ideas are combinations of existing ideas: one plus one equals three.