For an app to be functional, all you need to do is write your code correctly so that it does not crash. For an app to be a joy to use, though, you need to give it more love than that. You need to make it feel like a real, physical phenomenon playing out on a phone or tablet’s screen.

Real things move. To make your UI move, you animate its elements into new positions.

In this chapter, you will write an app called Sunset that shows a scene of the sun in the sky. When you press on the scene, it will animate the sun down below the horizon, and the sky will change colors like a sunset.