Pick your answer from:
(1). power.
(2). Chimneys.
(3). Sun.
(4). motion.
(5). Energy..
(6). sunlight.
(7). coal.
(8). energy.

Fact 1: Increasing heat causes thermal expansion. Fact 2: Motion helps to increase body heat. Given the two facts above, what causes thermal expansion?
Answer:
(4).