Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: To this point, we have examined the relationships between any two of the variables of , , and , while the third variable is held constant. However, situations arise where all three variables change. The combined gas law expresses the relationship between the pressure, volume, and absolute temperature of a fixed amount of gas. For a combined gas law problem, only the amount of gas is held constant. Answer: pressure

Answer: The combined gas law involves three properties of a gas - volume, absolute temperature, and what?

Question: Passage: Sperm are produced in the testes in the process of spermatogenesis. Answer: spermatogensis

Answer: What process creates sperm?

Question: Passage: Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian. Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt. It is the rock that makes up the ocean floor. Shown below are three types of extrusive igneous rocks ( Figure below ). Answer: gas bubble holes

Answer:
What helps pumice float in water?