Teeth are porous and made up of hollow tubelike structures called dentin tubules. These form the part of the tooth between the enamel (surface) and the pulp (core) of a healthy tooth. Some people develop stronger tooth enamel in their formative years, thanks to genetics and good brushing and flossing. But people with weaker enamel soak up things they eat and drink through their dentin tubules, and the surfaces of their teeth look stained.