Given a scientific passage and an answer, generate a question for the given answer.

[EX Q]: Passage: Potential energy is energy that is stored in a person or object. Answer: potential energy
[EX A]: Energy that is stored in a person or object is called?

[EX Q]: Passage: boundary between air masses that is stalled in one place. Answer: aids
[EX A]: Opportunistic diseases are usually the direct cause of death of people with what disease?

[EX Q]: Passage: The stirrup passes the amplified sound waves to the inner ear through the oval window (see Figure above ). When the oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid and lined with nerve cells called hair cells. Each hair cell has tiny hair-like projections, as you can see in Figure below . When the cochlea vibrates, it causes waves in the fluid inside. The waves bend the "hairs" on the hair cells, and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the nerve impulses reach the brain do we hear the sound. Answer: hair cells
[EX A]:
The cochlea is a shell-like structure that is full of fluid and lined with nerve cells called what?