A mule is the offspring of a male donkey and a female horse. Mules inherit interesting traits from both parents: they're typically stronger and more sure-footed than horses, yet calmer and more patient than donkeys. This combination made them invaluable working animals throughout history.

For centuries, mules hauled supplies across mountain trails, pulled plows on farms, and carried goods where wagons couldn't go. Their steady temperament and ability to navigate rocky terrain made them perfect for challenging work. During the California Gold Rush, mules transported miners and equipment through the Sierra Nevada Mountains. The Grand Canyon still uses mules to carry tourists down steep trails where vehicles can't travel.

Unlike horses or donkeys, mules almost never produce offspring of their own. This biological quirk means each mule must be deliberately bred rather than born from other mules.

The word also describes a stubborn person who refuses to change their mind, like someone being stubborn as a mule. This reputation for stubbornness isn't entirely fair to actual mules, which are intelligent animals that simply won't do things they consider dangerous or foolish. A mule won't overwork itself to exhaustion the way a horse might, which made them reliable partners for difficult jobs.