You can learn a lot of design from a sandwich
You're in your kitchen, making yourself a grilled cheese sandwich. A grilled cheese sandwich is more than just cheese and bread though, and you know this, so you have an egg, some ham, two kinds of cheese, fresh sour dough bread, and pickles. You fry the egg and set it aside. Melt some butter and lay the first piece of bread down. On goes the ham, the cheeses, the egg. You spread a healthy coat of mustard on the top piece of bread and gently place it on top. A few minutes and exactly one flip (not two, not three, ONE) later and you have your grilled cheese.
Your friend Jenny comes over in the middle of this process, and because it's lunch time, you offer to make her a sandwich too. She happily accepts and you double the above recipe. Perfection times two. She takes her first bite and asks honestly, "Where are the tomatoes? How can you make a grilled cheese without tomatoes?" You're very confused by this question, because clearly tomatoes aren't part of a grilled cheese. You've been making grilled cheese for years, a grilled cheese certainly has no tomatoes. While you eat your sandwiches, you decide to ask around.
You've been making grilled cheese for years, a grilled cheese certainly has no tomatoes.
At first you just ask your Facebook and Twitter friends, "Are tomatoes part of a grilled cheese?" and the vast majority of your friends answers, "Nooope!" The people have spoken. Jenny is wrong and grilled cheese certainly doesn't have tomatoes. End of discussion, right?
Jenny decides to ask your shared friends a different question, "What is on a grilled cheese sandwich?" And as you look over the responses you find that while there are no tomatoes on a lot of them, a lot of them are also missing egg. Some of them are just….ugg, bread and cheese. Is a sandwich just a subjective thing then? Can a grilled cheese be whatever we want it to be? Undefinable outside of the individual? Maybe not, for one thing all the recipes you are now getting in response to Jenny's question have things in common: they all have bread of some kind, cheese of some kind, and they're all hot. But why are they so different? So you ask some of your friends some other questions: "Why don't you have a fried egg on your sandwich?", "Why olive oil instead of butter?", "Why no ham?" and what you start to hear are answers like "I'm allergic to eggs", "Olive oil is cheaper than butter, but it browns the bread just as well", and "I'm a vegetarian."
Is a sandwich just a subjective thing then? Can a grilled cheese be whatever we want it to be? Undefinable outside of the individual? You start to see constraints and goals in the answers your getting. This is all very good information to have, because you were planning on having people over next week and now you can modify your sandwich to make sure you don't accidentally make a sandwich that costs too much, makes someone violate their ethics on eating meat, or maybe even sends them to the hospital.
You realize sandwiches aren't subjective, they're just fulfilling different needs and living inside different systems of constraints, and that's why they are different. And everyone loves a well thought out sandwich.