Languages express concepts via two types of meaningful sound: grammatical morphemes and lexemes. Grammatical morphemes are sometimes referred to as 'function words'. Grammatical morpheme are suffixes, prefixes, and unaccented particles that express the closed grammatical categories of language (Tense, Number, Gender, Aspect), each of which has one or more functions (Past, Present, Future are functions of Tense; Singular and Plural are functions of Number). Lexemes, on the other hand, are noun, verb, and adjective stems. They belong to open classes to which new lexemes may be added at will. They express general concepts like HOOD, IDEA, COOL, LONG, THINK, SLEEP. Lexemes, unlike grammatical morphemes, are also susceptible to derivation, e.g. sleep > sleep-er, cool > cool-ness.