Simple:
* The miners used mules to transport the ore out of the mine.
* The mules were strong and sure-footed, able to navigate the steep mountain trails.
Figurative:
* He was a mule when it came to listening to reason. (meaning stubborn)
* The smugglers used mules to carry drugs across the border. (meaning people who unknowingly transport illegal substances)
Specific:
* The mules were loaded down with sacks of grain, their backs groaning under the weight.
* Mules, a cross between a horse and a donkey, are known for their strength and hardiness.
Humorous:
* "Don't be a mule, just do what I say!" she shouted.
* The mule kicked the bucket, literally, when it got spooked by the passing train.
Remember to choose the sentence that best fits the context and your desired meaning.