Why do crocodiles swallow stones in the stomach?

Crocodile has a sharp teeth to chew on bait. But they also mimic birds, swallowing chickens in the stomach to digest food. Why so?

Crocodiles swallow the stone in the abdomen to ease the work for the stomach. Not only that, it also helps them balance their bodies. Four decades ago, British zoologists confirmed that. If there is no stone in the stomach, when swimming crocodiles very difficult to balance. Therefore, he had to use his swimming paws to beat the water so he could not be turned on his stomach.