Szydło's linguistic grounding for object-oriented messaging: in any language, a person can only build three kinds of sentences — a question, an order, or an informing statement. Accordingly, objects exchange only three kinds of messages: queries (ask), commands (order), and events (inform). The count isn't a programming constraint — it's a communication/linguistic one. Events enable the loosest coupling because the sender only declares what happened, leaving the 'what to do' to the receiver.