Ok, so this isn't really a 'proper' post but what the hey, I love this joke.
Idiot 1: 'Knock Knock!'
Idiot 2: 'Whose there?'
Idiot 1: 'Interrupting cow'
Idiot 2: 'Interrupting co...'
Idiot 1: 'MOOOOOOO!'
Hahahahahahahahahaha! I told my niece this joke on the weekend and then she told it back to me about fifty times in a row and each time we laughed so hard!
Who needs mad cow disease when you have jokes like this :D