Inhibiting and facilitating conditions of the human smile: A nonobtrusive test of the facial feedback hypothesis

Participants held a pen either between their teeth (which pulls the face into a smile) or between their lips (which makes a pout), under a cover story about doing tasks without using your hands. Then they rated how funny some cartoons were.

The teeth-pen group found the cartoons funnier, suggesting your face can tell your brain how to feel. Later replications have been more mixed, so the jury is still out, chewing thoughtfully on a biro.