Sometimes, we ask a question which elicits numerous answers. As you read those answers, you form related questions that don't probably merit entire new questions on their own but little one line answers as a follow-up to the original question.
The mechanisms for doing this today are using comments or posting a new question with a link to the original. Both of these are sub-optimal because they don't really show up in search, as being related. Further, modifying the original question to ask the follow-up query doesn't intimate the people who have already answered the original question and hence, loses its significance.
Should there be a way to post follow-up questions?