As in life, I believe that students should be "stimulated" by their surroundings... so when my advanced classes get a little too serious, I have a few MEAN stimuli in my repertoire of MEANNESS.  One of my favorites is the artificial reproduction of Methane in the human or "fart". This is especially effective when we're learning about the digestive system...I simply tape my remote fart machine to the bottom of an unsuspecting student's chair (usually the class clown or an outgoing personality).  While we are in discussion or quietly reading a passage, I secretly press the remote button. The reactions are great - first embarrassed silence and then a multitude of responses from the entire class as the unsuspecting student repeatedly looses control of his anal sphincter.  My machine has various lengths and amplitudes of sound....  great discussions and always chortling comments by the students especially "You're so Mean!"