To test out our function, we're first creating an array called stuff. In a playground file, click on the square box in the right pane and it will display the contents of the array (below the variable). The 0, 1, 2, and 3 numbers represent the index associated with the value at that index. As you've learned, arrays are indexed.  So, at index 0 you should find the value Coloring Book:

From what you've learned in this chapter, you should go through this function line by line to discern what it is that's actually happening and at what time. Give it your best shot! Look back at all the examples given in this chapter to help you. Don't rush through it either; it's important that you feel comfortable with these concepts. It's all about building more complexity on top of the basics, and if your core knowledge of Swift is strong, then it will make your further studies that much easier.

As you can see, validStuff is an array that contains all the same items that stuff has, except...