Some people can mould stories to fit a question and some find it very difficult, and I didn't say to go into the exam with nothing but dead flies and fluff in your brain - I said a few ideas.
I've read quite a few answers where people have had a whole story written up previously, gone into to exam and tried to fit it to the question, and totally wrecked the storyline and message in doing so.
Moulding is something like, say ... what they did in the Apollo 13 movie to get the CO2 levels down. They had a square filter to fit into a circular hole (or the other way around, I can't really remember), and they did this by adding other things and ended up with a big bulky thing that worked. However, it would have been way easier to just have a circle filter in the first place so it fits right in. I say it's easier to write a story from scratch with the question in mind ... but again, people are different, and they go about things differently. There's not just one correct way.