Until you want to model a bar-fight and then a chair (a piece of furniture) becomes a weapon, i.e. "class Chair: Furniture{}" turns into "Class Chair: Weapon {}". Or when the winter gets tougher and longer and "class Chair: Furniture{}" changes into "class Chair: Firewood{}". I know that there's probably a "solution" involving object "composition", but that would only mask the problem, will not really solve it.