Heat flows downhill. Which statement best describes this concept?

• A. It falls off hot surface to the floor.
• B. It naturally flows from a colder to a warmer substance.
• C. Heating ducts should flow downhill.
• D. It naturally flows from warmer to colder substance.

Answer: (D)

Heat flows from warmer to cooler substances because the temperature difference drives energy transfer until both objects reach the same temperature. This is the natural, downhill direction for energy, occurring by conduction, convection, or radiation. The statement that best describes this is that heat naturally flows from warmer to colder substance. The other options misrepresent the direction or mechanism: heat doesn’t physically fall, it doesn’t move from cold to hot without external influence, and ducts aren’t described by a downhill concept.