A millstone is a large, round stone used in the process of grinding grains into flour. Traditionally, two millstones are used: a stationary bottom stone and a rotating top stone. The grains are placed between the stones, and as the top stone turns, it crushes the grains into a fine powder. Millstones have been used for centuries in various cultures and are often made from durable materials like granite. They are an essential part of the milling process, which is crucial for producing food products like bread and pastries. The use of millstones has declined with the advent of modern milling technology.