Immutability. It may sound like an ominous something from the far-off galaxy of math, but in practice, it's one of the most pragmatic tools for thinking about UI. In this talk we'll explore the problems that an immutable style solves, and how you can use it as a thought-tool to both design and implement more powerful and composable components. Throughout we'll see just how deeply Ember supports this mode of thought at every step of the way.