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

Ex Input:
Passage: Optical telescopes are designed to collect visible light. The two types of optical telescopes are reflecting telescopes and refracting telescopes. Answer: reflecting and refracting

Ex Output:
What are the two types of optical telescopes?

Ex Input:
Passage: Hair, hair follicle, and oil glands. The oil, called sebum, helps to prevent water loss from the skin. The sebaceous gland secretes sebum, which waterproofs the skin and hair. Answer: sebum

Ex Output:
What does the sebaceous gland secrete which waterproofs the skin and hair?

Ex Input:
Passage: The freezing point of a substance is the temperature at which it freezes. The freezing point of pure water is 0C. Answer: freezing point

Ex Output:
What is the temperature at which a substance freezes known as?