Moo is the deep, low sound that cows make. If you've ever visited a farm or driven past a pasture, you've probably heard cows mooing to each other. The sound is distinctive and unmistakable: a long, bellowing “moooooo” that carries across fields.

Cows moo for different reasons. A mother cow might moo to call her calf. Cows moo when they're hungry and want to be fed, or when they're uncomfortable and need attention. Sometimes they seem to moo just to communicate with the rest of the herd, the way people might chat with their neighbors.

The word moo is what linguists call onomatopoeia: a word that sounds like what it describes. Just as buzz sounds like a bee and splash sounds like water, moo imitates the actual sound a cow makes. Young children learning animal sounds quickly learn that cows say “moo,” dogs say “woof,” and cats say “meow.”

You can use moo as a verb too: “The cows were mooing in the barn at feeding time.”