Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: The warm-air vent is placed near the floor of the room. Warm air is less dense than cold air so it rises. If the warm-air vent were placed near the ceiling instead, how would this affect the transfer of thermal energy throughout the room?. Answer: lower density
Solution:
What property of warm air causes it to rise above cold air?