An acorn is the fruit of an oak tree. It is a nut with one, or sometimes two seeds. The length of an acorn varies between 1 to 6 cm and it is 0.8 to 4 cm wide. Acorns cannot be stored because they do not retain their germination power for more than one season. They are an important food for animals which live near oaks, such as mice, squirrels, wild boars, deer and some ducks and other birds, as well as bears. For these animals, sometimes 25% of their winter supply consists of acorns. In the past, pigs were also fed with acorns (mast), but they are poisonous for horses.