There are situations where this doesn't work, like if you need a Vec that contains both Cats and Dogs, but for some cases it works great and doesn't require anything special at all.

Cat and Dog are both distinct types. They don't implement an Animal trait or anything. They just "happen" to have a method called `make_sound()` with the same signature, so when I sub one for the other, no code needs to change.

So excited for this! Another big idea for me coming from OOP languages was "syntactic polymorphism." What I mean is:

```
let animal: Cat = Cat::new();
animal.make_sound();
```

If later, I realize I need a Dog instead of a Cat:

```
let animal: Dog = Dog::new();
animal.make_sound();
```