(1)For thousands of years, humans have used certain animals to carry heavy loads. (2)The ox, the elephant, the donkey, and the mule are examples of these "beasts of burden." (3)Although these
animals have a reputation for being stubborn at times, they normally work very hard for long hours.
(4)One beast of burden, however, refuses to be overworked: the llama, a South American animal
much like a camel. (5)The llama knows just how much it can carry comfortably, and it will sit down
and refuse to budge if an extra half pound is placed on its back. (6)In addition, a llama will carry a
burden only a certain distance. (7)After it travels twenty miles, it sits down in the middle of the
road: nothing will convince it to continue. (8)After the animal has decided to quit, its owner
sometimes tries to prod it with a stick. (9)When it is annoyed in this fashion, the llama has an
unusual way of striking back. (10) It puckers its lips and spits in the owner's face.