Students conduct an investigation to explore the process of heat transfer. The students use a hot plate, a pan, water, and a piece of uncooked elbow macaroni. The students place the pan of water on the hot plate and observe what happens to the macaroni noodles as the water heats. As the water temperature rises, the students observe the macaroni noodles rising and sinking in a circular motion. The students are most likely performing a descriptive investigation about heat transfer by –