Laughter is contagious, and when we hear it, our brain automatically wants to get in on the action. According to multiple studies, including studies at the Institute of Cognitive Neuroscience, University College London, we naturally mimic the gestures and facial expressions of those we talk to, a trait that is now also thought to include laughing [source: ScienceDaily]. We laugh when we hear a funny joke, when we play, when we're tickled … but laughing isn't just an indication we think something is funny. It's also one of the ways we bond with other people. Laughter is universal and a social lubricant, so go ahead and see that new Seth Rogen movie on date night -- not only will your laughter boost your mood, but it just might boost your bonding.