Linguistics is the scientific study of human language. It explores the unconscious knowledge that speakers have about language, in particular the relationship between sound and meaning, which is at the core of language as a system of communication. Linguists study the structure and representation of language at various levels: sounds (phonetics and phonology), words (morphology), phrases and sentences (syntax), and meaning (semantics). They also investigate questions about language universals and typology, the historical development and changes in language over time, the acquisition of language by children and adults, and the processing of language in the mind/brain. Linguistics is an interdisciplinary field that investigates language from theoretical, descriptive and experimental perspectives.