Chickens like to peck around on the ground, searching for food. In addition to eating edible items, they often pick up sand grains or small stones to eat. Is it because the chickens are too hungry, or did they accidentally mistake the stones for food? It seems that neither of these is the case, because even after the chickens are full, they still look for sand grains and small stones to eat. So, what's going on here? It turns out that chickens don't have teeth. They don't chew their food but swallow it whole. However, food needs to be crushed before it can be digested. Therefore, chickens peck at some sand and stones to help digest their food.
Not just chickens, many birds have this "habit". Especially those birds that, like chickens, eat hard foods like seeds. They first store the food in a place called the crop located in the throat, and then slowly transport it to the stomach, relying on small stones to crush it and further digest and absorb it. Most birds' stomachs consist of two closely connected parts: the foregut and the hindgut. The foregut is softer and filled with glands, where gastric juice is secreted. The walls of the hindgut are thicker, with many muscles attached, and a hard, wear-resistant cuticle on the surface. In a chicken's stomach, this layer is the familiar "chicken gold" in the chicken gizzard. The hindgut is where stones are used to crush food, so it's also called the gizzard.
Of course, not all birds rely on stones to crush food. Carnivorous birds rely more on glands and bacteria to break down and digest food. And birds that eat fruits often don't even have a distinct stomach, only a straight digestive tract, where digestion and absorption are completed directly as the food passes through the tract.