> A vehicle can have any number of wheels and any number of doors. Does that mean zero doors and zero wheels? Because a jet ski is surely a vehicle, right, and

> A vehicle can have any number of wheels and any number of doors. Does that mean zero doors and zero wheels? Because a jet ski is surely a vehicle, right, and it has zero wheels and zero doors. Likewise, what about a cable car (aerial lift)? It has doors (maybe), but probably not an internal engine. It's definitely a vehicle, but it has almost nothing that's similar about it compared to the car (automobile) you described earlier. If it where a class, the "vehicle class" could really only meaningfully have some attributes related to transport (like human/cargo capacity) and some methods around movement in the most general sense. If we keep breaking this down, we'll find a huge taxonomical network, a vast sea of interlocking Venn-diagrams of attributes and capabilities. This is the corner that those OOP tutorials force people into: trying to turn a network into a tree, and it doesn't ever really work (at least not without awkward compromises). Sure, for a beginner who's never grappled with trying to map concepts to things, this can be a nice contained way to start. Eventually though, that beginner will need to learn some the next levels of more general thinking; e.g. one where they can start modeling interfaces/protocols and their relationships.