Informal vs. formal is one form of juxtaposition that can be funny. Another form can be using thoughts or words inappropriate to the speaker. Can you picture Abe Lincoln singing “The Camptown Races?” The Look Who’s Talking movies gave babies adult voices and thoughts. Another juxtaposing pair can be beauty vs. ugliness. Such juxtapositions are like the repetition of the villanelle, they heighten the effect, whether comic or tragic.