Flatworms are hermaphrodites, meaning they have both male and female genitals. This can make mating a little tricky when trying to figure out who gets to be the male and who gets to be the female. Most flatworms are inclined to play the male role, and they settle things with a good old fashioned sword fight -- with their penises. Scientists actually call this duel "penis fencing." The loser ends up getting stabbed with the winner's penis and absorbs its sperm, becoming pregnant.