The Input Hypothesis, proposed by Dr. Stephen Krashen, suggests that language development is a function of the input received by the learner. Krashen asserts that there are two distinct ways of learning language: language "learning" and language "acquisition". Language "learning" is learning that takes conscious effort on the part of the learner. It is characterized by learning grammar rules, memorizing vocabulary lists, and performing speaking drills. Language "acquisition" is learning that is subconscious and takes little or no effort on the part of the learner. It is characterized by listening to and understanding messages, reading interesting books and articles, and other enjoyable activities that take place in the language being learned. According to Krashen's theory, the only thing that can lead to fluency in the language is language "acquisition". Language "learning" can only be used as a way to consciously edit speech or writing, and it is never the cause of spontaneous, unrehearsed speech or writing.