Suppose you write a method that takes two inputs and returns the sum. What would you do next? You'd probably do something like you just suggested, verify that

Suppose you write a method that takes two inputs and returns the sum. What would you do next? You'd probably do something like you just suggested, verify that when you add 2 and 2 here, you get 4. So, save it. This isn't a complete or perfect test. In fact, this particular test is risky, because later, when someone decides that addify should multiply, your tests will still pass. I'm not going to pretend that this solves your problem. But if you had written 5, 2, and 3, it would have protected you from that later scenario. And IMO, it's not the dev's fault who turned addify into a multiplication function - how is he or she supposed to know it broke the code if there aren't any tests? To me, though, the bigger problem is when you're trying to figure out what your code is supposed to do by writing code, and then shoehorn that process into TDD. Like, write down 42, then make it fail, and then make it pass.