A very short introduction to Optimality Theory, a linguistic framework that attempts to account for the grammar of human languages. Traditional models of grammar rely on complementary, unbreakable rules. The outcomes (sounds, words and sentences) follow those rules. Optimality Theory models grammar with ranked, violable, competing constraints. The outcomes represent the best candidates - the ones incurring the least serious violations. This video uses the analogy of two friends making plans for the night. One employs rule-based reasoning, the other constraint-based reasoning. The video then goes on to analyze the simplified grammar of plural -s (pronounced [s] in 'hats' but [z] in 'bags') using an Optimality Theory tableau. For an academic consideration of constraints, rules, optimal candidates, markedness/faithfulness and tableau conventions, refer to Prince & Smolensky, Optimality Theory: Constraint Interaction in Generative Grammar, sections 1.1, 1.2 and 2.2 (15). Music by Kevin MacLeod