Find the boundary of an idea using counterexamples

Teach [CONCEPT] to [LEARNER LEVEL] by first stating a provisional rule in plain language, then offering three examples and three carefully chosen near-miss counterexamples. Provide a learner-facing sequence that asks for classification and reasoning before revealing any answer. In a separate instructor key, refine the rule, give answer reasoning and common traps, and add one new independent case. Do not show the key during interactive practice. Avoid exceptions that rely on obscure trivia unless supplied by the teacher. Check that each counterexample really violates the initial rule and that the refined version handles all six cases.