^It *could* have been the lazy sperm: the one that was busy
   partying the night before and while all the other sperm were
   weakening the egg shell, this little fella was stumbling its way
   up the fallopian tubes, tripping over the other sperm. He looks
   around like, "Man, all my brothers are DEAD man, what's
   happening around here! I gotta get out of here!", when suddenly
   he sees a little cave, "Oh, that looks like a nice place to
   sleep off this hangover." and.... viola! Happy Birthday!^