They are sometimes quite cheeky, but the point is not to teach the concept but to have fun, provoke the students gently and trigger verbal exchanges. Even the most humble of statements will very often lead on to further talk, through opinion, disagreement, comment, adding to, agreeing and so on. Married people are boring. Footballers are not intelligent. You can't be friends with your boss.
Old people have no fun.
Men are bad at languages. Young girls are brighter than young boys. Engineers and accountants are very rational people. Mathematics are for intelligent people. All politicians are corrupt.