If someone tells me something really funny I tend to slap my hand of my knee, or just give a single clap with my hands - it's never more than one slap/clap. I've noticed my friends doing it too. It's like the human version of "bad dum tish" after a joke. Do you do it too? Can't find anything on Google! Would anyone care to do a bit of an analysis as to why us humans do this?