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

Q: Passage: Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. That’s why it’s easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil. The pencil “lead” slides easily over the paper, but there’s just enough friction between the pencil and paper to leave a mark. Answer: sliding friction

A: What type of friction is friction that acts on objects when they are sliding over a surface?
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Q: Passage: Experiments are sometimes hard or even impossible to do. For example, a scientist who is studying an extinct animal cannot experiment with the animal because it no longer exists. The scientist must rely instead on evidence in the natural world, such as fossils that the extinct animal left behind. Answer: experiments

A: What can be hard or even impossible to perform in a scientific sense?
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Q: Passage: The solubility of gases is also affected by pressure. Pressure is the force pushing against a given area. Increasing the pressure on a gas increases its solubility. Did you ever open a can of soda and notice how it fizzes out of the can? Soda contains dissolved carbon dioxide. Opening the can reduces the pressure on the gas in solution, so it is less soluble. As a result, some of the carbon dioxide comes out of solution and rushes into the air. Answer: increase

A:
Increasing the pressure on a gas has what effect on its solubility?
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