Here's a good example... one candidate chose to explain how residential firefighting works. They got into explaining why water is really important. I asked them about halon and they indulged a little bit. Then I asked them to explain how halon works. And so on...

If they haven't shut me down by the 3rd question, it's a sign they're never going to, and I stop distracting them.

For junior candidates, I usually don't even bother with this, because I am not going to stop myself from hiring just because they fail this question. So it's usually better to see if they can do well at just the core question.