Passage Summary: You have to be at least eighteen to be a professor or a voter. Brilliant people, however, show up in all three groups: some are professors, some are voters, and some are still under eighteen.

Reasoning: The passage establishes that professors and legal voters are exclusively eighteen or older, while the category of 'brilliant people' includes some professors, some legal voters, and some individuals under eighteen.

Analysis: In 'Must be True' questions, we treat the stimulus as a set of ironclad rules and look for what inevitably follows from their intersection. We know that being under eighteen is an absolute bar to being a professor or a voter; since some brilliant people are under eighteen, it follows that these specific brilliant individuals are definitely not professors or legal voters. Conversely, we know that the brilliant people who are professors must be at least eighteen years old. Look for an answer choice that synthesizes these categories, such as 'some brilliant people are not professors' or 'some brilliant people are not legal voters.'