Answer and Explanation:
Holding a pen between their teeth was inducing participants to smile. Or, at least, that is how their brains seemed to interpret it. Their face muscles were tensed in a way that resembled a smile, which caused positive facial feedback. Their brains interpreted that "smile" as a sign of a positive feeling, which explains why the participants rated the comics as funnier.
On the other hand, when participants were asked to hold the pen between their lips, their smiles were inhibited. Their lips were tense in a completely different way, as if they were pouting. While doing so, they rated the comics as less funny, since their brains interpreted their pouting as a sign of a negative feeling.