Today, the huge, airy farm structures we know as barns are used mostly to store modern farm machinery and house farm animals. But before modern farming, they had a greater number of important uses. Before the invention of threshing machines (which separate cereal grains like wheat from their stalks), the grain harvest had to be stored in barns, where it would await threshing or pounding by hand during winter months. The structures had to be large and drafty for the process of winnowing, which separated straw dust from the grains after threshing. Before farmers began to raise special crops to feed their livestock during the winter, they used hay, which is dried grass (grown wild or taken from the stalks of cereal crops). Huge amounts—enough to last several months—had to be stored away. Hay was usually kept in barn lofts located above the main floor, where farm animals spent the winter. This high storage place allowed air to circulate around the hay, keeping it from rotting. It was convenient, too, because hay could be pulled down as needed to feed the livestock. Because farmers had to store their harvest crops in barns, they wisely cut entrance holes near their roofs, inviting barn owls to make nests there. The birds would hunt the rats and mice that liked to feed on the grain.