Well, rhetoric comes from ancient Greece, where it was an art of public speaking designed to persuade the listener. For a "rhetorical question", it's a question that doesn't need an answer because the answer is implied. For example, "What normal person would ever drive a car off a cliff?" We don't need to answer that because we know normal people don't drive cars off cliffs. So if you're talking about someone who did drive a car off a cliff, you're implying that person is not normal. To create a rhetorical question, you take a statement of your argument ("Normal people don't drive cars off cliffs") and present it in question form. Of course the rhetorical question won't always be perfectly logical, or withstand analysis or examples, but it's used to persuade the listener of your argument due to its emotional impact.