Oak trees produce the fruit commonly called an acorn.
An acorn is the nut-like fruit of an oak, a tree in the genus Quercus. It usually sits in a distinctive cup-shaped structure called a cupule, which is formed from modified plant tissues. Acorns vary in size and shape among oak species, but their familiar cap is a useful field clue.
Acorns are important food for wildlife such as squirrels, deer, wild turkeys, jays, and woodpeckers. Many oak species produce a large crop only periodically, a pattern called masting. Humans have also eaten acorns after processing them to reduce bitter tannins. A common mix-up is calling every capped woodland nut an acorn; beech and chestnut fruits are different and do not come from Quercus trees.