You can't linke direct to TED Videos here (damn it) so you have to follow the link I guess. TED is an awsome site with many intressting and sometimes funny talks. This one is very intressting for the FJ-Community I guess but see for yourself.
Kevin Allocca is YouTube's trends manager, and he has deep thoughts about silly web video. In this talk from TEDYouth, he shares the 4 reasons a video goes viral.