A medieval castle had been under siege for three months. Supplies of food and water were virtually exhausted, leaving the occupants in dire straits. Unless aid arrived soon, everyone would perish.
Realizing the gravity of the situation, one of the knights offered to break out of the castle and ride for help. Alas, all the horses had been killed so, in desperation, the knight suggested riding out on the back of a huge wolfhound.
'You can't,' said the keeper of the dogs, 'the wolfhound has injured its leg. The only dog available is this Chihuahua, and I wouldn't send a knight out on a dog like this!'