This document discusses second language acquisition (SLA). SLA refers to learning a language after one's native language. The goals of studying SLA are to describe how L2 is acquired and explain the process, identifying external and internal factors influencing acquisition. External factors include social environment and input, while internal factors include cognitive mechanisms and communication strategies. The document presents case studies of an adult learning English and two children learning English, showing their progress in requests over time. It also discusses methodological issues in describing and explaining L2 acquisition.