Mary is a vegetarian. She only cooks and eats vegetarian food. John is carnivorous. He cooks and eats meat! If Mary wants to eat broccoli and cheese, she can learn how to cook broccoli and cheese. If she wants corn of the cob, she can learn how to cook corn on the cob. The same goes for John. If he wants to eat greasy hamburger, he can learn how to cook greasy hamburger. If he wants to eat fatty hotdog, he can learn how to cook fatty hotdog. Every time John and Mary want to eat something new, they can learn how to cook it. This requires that John and Mary to each have a very big head in order to learn all the recipes.
But wait, there are people out there called chefs! These are very special kinds of chefs catering only to vegetarians and carnivores. These chefs only know how to cook two dishes: one vegetarian dish and one meat dish. All John and Mary have to do is to know how to ask such a chef to cook their favorite dish. Mary will only order the vegetarian dish, while John will only order the meat dish! How do we model the vegetarian, the carnivore, the chef, the two kinds of dishes the chef cooks, and how the customer orders the appropriate kind of dish from the chef?