Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: The Universe contains about a hundred billion galaxies. Answer: about a hundred billion

Answer: Approximately how many galaxies are there in the universe?

Question: Passage: Classification of Joints on the Basis of Function The functional classification divides joints into three categories: synarthroses, amphiarthroses, and diarthroses. A synarthrosis is a joint that is immovable. This includes sutures, gomphoses, and synchondroses. Amphiarthroses are joints that allow slight movement, including syndesmoses and symphyses. Diarthroses are joints that allow for free movement of the joint, as in synovial joints. Answer: synarthrosis

Answer: What category of joints is immovable?

Question: Passage: Density is a quantity that expresses how much matter is packed into a given space. The amount of matter is its mass, and the space it takes up is its volume. To calculate the density of an object, then, you would use this formula:. Answer: density

Answer:
What is the term for how much matter is packed into a given space?