Omar makes a model of how tectonic plates move. He fills a metal tray with water and places a spirit burner under the middle of the tray. He floats two small pieces of polystyrene side by side on the water in the middle of the tray, then times how long they take to drift apart and reach the two ends of the tray. As soon as they reach the ends, he moves them back to the middle and repeats, using the same water. His results: run 1 = 31.6 s; run 2 = 27.9 s; run 3 = 23.0 s; run 4 = 19.4 s; run 5 = 15.1 s. Explain what makes the pieces of polystyrene move.