Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Kevin Walsh. Cows are able to digest grass with the help of the bacteria methanogens in their gut . CC BY 2.0. Answer: bacteria

Answer: A type of what in a cow's gut enables the animal to digest grass?

Question: Passage: Natural gas is delivered to homes, where it is used for cooking and heating. Natural gas is also a major energy source for powering turbines to make electricity. Natural gas releases most of its energy as heat when it burns. The power plant is able to use this heat, either in the form of hot gases or steam, to spin turbines. The spinning turbines turn generators, and the generators create electricity. Answer: natural gas

Answer: What type of gas is delivered to homes to use for cooking and heating?

Question: Passage: Metamorphism. This long word means "to change form. " A rock undergoes metamorphism if it is exposed to extreme heat and pressure within the crust. With metamorphism, the rock does not melt all the way. The rock changes due to heat and pressure. A metamorphic rock may have a new mineral composition and/or texture. Answer: heat and pressure

Answer:
What is a rock exposed to if it's undergoing metamorphism?