Never mind if people are smart or stupid. The important thing for a forum is if the participants are willing and able to provide thoughtful and interesting comments. Simple quizzes like the above (joke or not) might filter out a lot of lazy trolls up front. Everybody with a genuine interest in the forum should easily be able to find the answers using Google, and might even find it fun to learn a bit.

More importantly, after having made a bit of effort to be allowed in, the users might value the forum more.