also been used to explain errors in SLA, as the creation of supersets could signal over-generalization, causing acceptance or production of ungrammatical sentences. Pienemann's teachability hypothesis is based on the idea that there is a hierarchy of stages of acquisition and instruction in SLA should be compatible with learners' current acquisitional status. Recognizing learners' developmental stages is important as it enables teachers to predict and classify learning errors. This hypothesis predicts that L2 acquisition can only be promoted when learners are ready to acquire given items in a natural context. One goal of learnability theory is to figure out which linguistic phenomena are susceptible to fossilization, wherein some L2 learners continue to make errors despite the presence of relevant input. Variability Although second-language acquisition proceeds in discrete sequences, it does not progress from one step of a sequence to the next in an orderly fashion. There can be considerable variability in features of learners' interlanguage while progressing from one stage to the next. For example, in one study by Rod Ellis, a learner used both "No look my card" and "Don't look my card" while playing a game of bingo. A small fraction of variation in interlanguage is free variation, when the learner uses two forms interchangeably. However, most variation is systemic variation, a variation that depends on the context of utterances the learner makes. Forms can vary depending on the linguistic context, such as whether the subject of a sentence is a pronoun or a noun; they can vary depending on social contexts, such as using formal expressions with superiors and informal expressions with friends; and also, they can vary depending on the psycholinguistic context, or in other words, on whether learners have the chance to plan what they are going to say. The causes of variability are a matter of great debate among SLA researchers. Language transfer One important difference between first-language acquisition and second-language acquisition is that the process of second-language acquisition is influenced by languages that the learner already knows. This influence is known as language transfer. Language transfer is a complex phenomenon resulting from the interaction between learners’ prior linguistic knowledge, the target-language input they encounter, and their cognitive processes. Language transfer is not always from the learner’s native language; it can also be from a second language or a third. Neither is it limited to any particular domain of language; language transfer can occur