Well-chosen examples do more than decorate an explanation: they show how an abstract principle operates under real conditions. In education, a worked example can reduce cognitive load by making each decision visible, while a contrasting case reveals where the same rule no longer applies. A clear test case demonstrates this distinction by showing how a model performs with ordinary inputs, edge conditions, and incomplete information. The surrounding explanation should identify the relevant variables, since examples become misleading when readers cannot tell which details matter and which are incidental. Practical writing therefore benefits from a sequence that moves from a simple illustration to a realistic application and then to a brief exception. This structure supports understanding without pretending that one case represents every possible outcome.