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

Example Input: Passage: Earth's climate has changed many times. Long warm periods were broken up by ice ages. Over the past 150 years, climate has warmed quickly. Answer: ice ages
Example Output: What occurs between long warm climate periods?

Example Input: Passage: When a fluorine atom gains an electron, it becomes a negative fluoride ion. Answer: fluoride ion
Example Output: When a fluorine atom gains an electron, it becomes a negative what?

Example Input: Passage: Chemical sedimentary rocks form when crystals precipitate out from a liquid. The mineral halite, also called rock salt, forms this way. You can make halite! Leave a shallow dish of salt water out in the Sun. As the water evaporates, salt crystals form in the dish. There are other chemical sedimentary rocks, like gypsum ( Figure below ). Answer: chemical sedimentary rocks
Example Output:
What are formed when crystals precipitate out from a liquid?