Which nut is depicted in the picture?

The acorn, or oaknut, is the nut of the oaks and their close relatives. It usually contains one seed (occasionally two seeds), enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns are 1–6 cm (1⁄2–2+1⁄2 in) long and 0.8–4 cm (3⁄8–1+5⁄8 in) on the fat side. Acorns take between 6 and 24 months (depending on the species) to mature.

Wildlife that consume acorns as an important part of their diets include birds, such as jays, pigeons, some ducks, and several species of woodpeckers. Small mammals that feed on acorns include mice, squirrels and several other rodents.

Large mammals such as pigs, bears, and deer also consume large amounts of acorns; they may constitute up to 25% of the diet of deer in the autumn.

Acorns are too heavy for wind dispersal, so they require other ways to spread. Many animals eat unripe acorns on the tree or ripe acorns from the ground, with no reproductive benefit to the oak, but some animals, such as squirrels and jays serve as seed dispersal agents. Jays and squirrels that scatter-hoard acorns in caches for future use effectively plant acorns in a variety of locations in which it is possible for them to germinate and thrive.

Acorns served an important role in early human history and were a source of food for many cultures around the world. Acorns have frequently been used as a coffee substitute, particularly when coffee was unavailable or rationed.