I read an article once (and I wish I'd saved that thing) about a study done about this very subject.
The gist of it was that stupid people think themselves quite intelligent, and have a need to let everyone know. Also that stupid people are too stupid to realize just how stupid they are, let alone that they're stupid in the first place. Even if proof is presented to them, they'll deny it to their dying day. I suppose that ignorance really is bliss.
Here's one: (I've actually been asked this and they were actually serious)
What's the number for 911?