Famous 1981 science fiction/horror film (http://en.wikipedia.org/wiki/Scanners) directed by David Cronenberg in which people have the psychic ability to make other people's heads explode. When he's angry with Leonard, Sheldon puts his fingers to his forehead and glares, mimicking the psychic attack used by the people in the film. Fortunately, Leonard's head does not explode.
During lunch in the cafeteria one day in early 2009, Sheldon is about to rattle off one of his litanies of random facts, in this case about tapioca pudding, when Howard interrupts intentionally, followed by Raj and Leonard. They're doing it on purpose, and he grimaces from the exertion of not being able to say what was on his mind. Finally, Leonard tells the others they they'd better let him say what he has to say "before his head explodes", a reference to the film "Scanners". (0213)