This action produces thermal energy through friction. This action conducts thermal energy away from the body. This action captures thermal energy from the environment. This action reduces the amount of thermal energy transferred to the air.
I gave my students this multiple choice question: A student stands outside on a cold winter day. His hands become cold and he rubs them together to make them warmer. Which statement explains why rubbing his hands together makes them warmer?

Only one answer is correct among these 4 choices:
- This action produces thermal energy through friction.
- This action conducts thermal energy away from the body.
- This action captures thermal energy from the environment.
- This action reduces the amount of thermal energy transferred to the air.

Could you tell me which one is correct?

This action produces thermal energy through friction.