Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: 16.3 Circulatory and Respiratory Systems Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humidified in the nasal cavity. Air then travels down the pharynx and larynx, through the trachea, and into the lungs. In the lungs, air passes. Answer: nasal cavity

Answer: In mammals, air is warmed and humidified in what?

Question: Passage: Figure 32.21 Supernovas are the source of elements heavier than iron. Energy released powers nucleosynthesis. Spectroscopic analysis of the ring of material ejected by Supernova 1987A observable in the southern hemisphere, shows evidence of heavy elements. The study of this supernova also provided indications that neutrinos might have mass. (credit: NASA, ESA, and P. Challis). Answer: supernovas

Answer: What celestial events are the source of elements heavier than iron?

Question: Passage: Remember that a phase change depends on the direction of the heat transfer. If heat transfers in, solids become liquids, and liquids become solids at the melting and boiling points, respectively. If heat transfers out, liquids solidify, and gases condense into liquids. Answer: heat transfer

Answer:
What type of direction does a phase change depend on?