Say what you actually picture, and then say what the picture feels like — that second half is what the teacher is listening for.
Sample answer: The first thing I think of is the first heavy rain, when the drain in front of our house turns into a stream and every child in the lane comes out with a torn notebook page. I think of the careful folding, the wobble as the boat is set down, the race to keep up with it, and the sad soggy moment when it tips over. A paper boat also makes me think of how little you need in order to play — no money, no toy shop, just one sheet of paper and some running water.