Phrase structure rules break down natural language sentences into constituent parts such as phrases and lexical categories. They operate based on constituency and are used in constituency grammars. Phrase structure rules take the form A->BC, indicating a constituent separates into two subconstituents. Examples show rules for English sentences and phrases. Applying phrase structure rules can generate syntactically correct but semantically nonsensical sentences, as demonstrated by Noam Chomsky's example "Colorless green ideas sleep furiously."