Over the years, a great deal has been written about the ontology of pizza ( http://robertdavidstevens.wordpress.com/2010/01/22/why-the-pizza-ontology-tutorial/ ). It’s a good example, is easy to understand and works surprisingly well in a tutorial context. It is also comes up surprisingly commonly in the public sphere as it did last year on BBC News ( http://www.bbc.co.uk/news/magazine-33542392 ). The key point of which is this: the pizza maker argues that you can’t have a marinara (tomato and garlic) with added mozzarella because a marinara is pizza rossa which can’t have mozzarella; a margherita (tomato and mozzarella) with garlic is fine though. Ha, those crazy Italians. I paraphrase, of course.