Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Gases in the coma and tail of a comet reflect light from the Sun. Comets are very hard to see except when they have comas and tails. That is why they appear only when they are near the Sun. They disappear again as they move back to the outer solar system. Answer: comas and tails
A:
What parts of comets make them easy to see?