Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: When water freezes in a non-crystalline, amorphous form as frost, the water molecules are fixed in place, but not in a regular, repeating pattern. Frost and other amorphous solids are characterized by their unorganized and unpredictable structure. Because ice comes in both amorphous and crystalline forms, there are differences in the physical properties of ice such as its hardness and density. Answer: amorphous

Answer: What kind of solid is characterized by an unorganized and unpredictable structure?

Question: Passage: An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another. Answer: variable

Answer: In a controlled scientific study, what do you call a factor that can take on different values?

Question: Passage: The other idea is that evolution occurs by natural selection. Natural selection is the process in which living things with beneficial traits produce more offspring than others do. This results in changes in the traits of living things over time. Answer: natural selection

Answer:
What is the term for the process in which living things with beneficial traits produce more offspring than others do?