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

Example Input: Passage: The epidermis is the outer layer of skin, consisting of epithelial cells and little else (see Figure below ). For example, there are no nerve endings or blood vessels in the epidermis. The innermost cells of the epidermis are continuously dividing through mitosis to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing a tough, fibrous protein called keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer called the stratum corneum. The dead cells are gradually shed from the surface of the skin and replaced by other cells. Answer: epidermis
Example Output: What is the outer layer of skin, consisting almost entirely of epithelial cells?

Example Input: Passage: Any magnet has two ends called poles where the magnetic effect is strongest. Answer: poles
Example Output: What do you call the two ends of a magnet where the magnetic effect is strongest?

Example Input: Passage: Natural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation. This causes allele frequencies to change. Answer: natural selection
Example Output:
What occurs when there are differences in fitness among a population?