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

[EX Q]: Passage: Figure 29.17 The tails of the Hale-Bopp comet point away from the Sun, evidence that light has momentum. Dust emanating from the body of the comet forms this tail. Particles of dust are pushed away from the Sun by light reflecting from them. The blue ionized gas tail is also produced by photons interacting with atoms in the comet material. (credit: Geoff Chester, U. Navy, via Wikimedia Commons). Answer: momentum
[EX A]: The tails of the hale-bopp comet point away from the sun, evidence that light has what property of motion?

[EX Q]: Passage: Heat waves have increased in frequency and duration in recent years. The summer 2011 North American heat wave brought record temperatures across the Midwestern and Eastern United States. Many states and localities broke records for temperature and for the most days above 100 o F. Answer: cell membrane
[EX A]: What are all cells surrounded by?

[EX Q]: Passage: Reflection of light occurs when light bounces back from a surface that it cannot pass through. If the surface is very smooth, the reflected light forms an image. Answer: reflection
[EX A]:
What is the process where light bounces back from surfaces that it cannot pass through?