Why do birds eat grit?

Birds which peck grain and other seeds also peck grit to help them to digest these hard foods. Because birds have no teeth, the work of chewing, which would require muscles and strong jaw bones,is done by the gizzard. grit is taken into the gizzard,or ventriculus,which has thick and often very muscular walls and the combined action of the two grinds down the hard food.(questions and answers)