Solemn Bart is not a fun Bart. It's actually a very boring, infuriating Bart. And so, Jenny has an idea.

Two can play at this game. She nods, now equally solemn rather than continuing her with her pout. The change in expression ought to be fearful. But before her boyfriend has a chance to question, she answers:

"Well, I think you have a very nice penis."

And as she states such, Jenny reaches downward to cup said anatomy as she pushes him back against the booth.