Proponents of TPR Storytelling, basing their argument on the second language acquisition theories of Stephen Krashen, hold that the best way to help students develop both fluency and accuracy in a language is to expose them to large amounts of comprehensible input. The steps and techniques in TPR Storytelling help teachers to provide this input by making the language spoken in class both comprehensible and engaging. In addition, TPR Storytelling uses many concepts from mastery learning. Each lesson is focused on just three vocabulary phrases or fewer, enabling teachers to concentrate on teaching each phrase thoroughly. Teachers also make sure that the students internalize each phrase before moving on to new material, giving additional story lessons with the same vocabulary when necessary.