Linguists generally accept nine English word classes: nouns, verbs, adjectives, adverbs, pronouns, prepositions, conjunctions, determiners, and exclamations. English words are not generally marked for word class. It is not usually possible to tell from the form of a word which class it belongs to except, to some extent, in the case of words with inflectional endings or derivational suffixes. On the other hand, most words belong to more than one-word class. For example, "run" can serve as either a verb or a noun (these are regarded as two different lexemes). Lexemes may be inflected to express different grammatical categories. The lexeme "run" has the forms "runs", "ran", "runny", "runner", and "running". Words in one class can sometimes be derived from those in another. This has the potential to give rise to new words. The noun "aerobics" has recently given rise to the adjective "aerobicized".