Given a scientific passage and an answer, generate a question for the given answer.
--------
Question: Passage: Bacteria are so small that they can only be seen with a microscope. When viewed under the microscope, they have three distinct shapes ( Figure below ). Bacteria can be identified and classified by their shape:. Answer: by their shape

Answer: How are bacteria identified and classified?

Question: Passage: Cells are the tiny building blocks of living things. They couldn't be discovered until the microscope was invented. The discovery of cells led to the cell theory. This is one of the most important theories in life science. Answer: cells

Answer: What are all living things made of?

Question: 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

Answer:
The tails of the hale-bopp comet point away from the sun, evidence that light has what property of motion?