The purpose of this article is to describe a modeling sequence about the digestive system. "What happens in your body when you have lactose intolerance?" is the initial question that will guide the activities. Students will build a 3D plaster cast of the digestive system as part of a modeling cycle through which they are expected to evaluate and refine their models both individually and cooperatively. Today, about two-thirds of the population in the world are known to have lactose intolerance. Besides, lactose intolerance and allergy to cow's milk are usually mistaken by the public. For those reasons, the topic seems fundamental to be taught to children in the science class.