The Triceratops lacked suitable teeth for grinding food and so, like birds, would swallow rocks and use them as gizzard stones. In the digestive tract, these rocks would grind the food to aid in digestion. After six weeks, the rocks would become too smooth to be useful, and the animal would regurgitate them. When finding and eating new rocks to use, the animal would also swallow West Indian Lilac berries. The fact that the berries and stones are regurgitated explains why Dr. Ellie Sattler never finds traces of them in the animal's excrement.