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

[Q]: Passage: Matter can be classified as transparent, translucent, or opaque depending on how it interacts with light. Answer: opaque
[A]: Depending on how matter interacts with light, it can be classified as transparent, translucent, or what?

[Q]: Passage: To harness water power, a stream must be dammed. Narrow valleys are the best for dams. While sitting in the reservoir behind the dam, the water has potential energy. Water is allowed to flow downhill into a large turbine. While flowing downhill, the water has kinetic energy. Kinetic energy makes the turbine spin. The turbine is connected to a generator, which makes electricity. Answer: kinetic energy
[A]: Water is allowed to flow downhill into a large turbine to harness power. what helps the turbine spin?

[Q]: Passage: The mechanical advantage of a simple machine such as a pulley is the factor by which the machine changes the force applied to it. The ideal mechanical advantage of a machine is its mechanical advantage in the absence of friction. All machines must overcome friction, so the ideal mechanical advantage is always somewhat greater than the actual mechanical advantage of the machine as it is used in the real world. Answer: friction
[A]:
What is something that all machines must overcome?