animal that is the offspring of a donkey and a horse,used for carrying loads and noted for its stubbornness
slipper that is open around the heel
A good mule can work two horses into the ground.
出自： W. van T. Clark
Obstinate as a mule over that, he was.
出自： J. Wyndham
Pink feathered mules on her feet.
出自： M. Wesley