I did the Car with Rotating Wheel task, but when I first made the Rotating Wheel symbol, I forgot to specify the default symbol behavior as Movie Clip. I went back to the Library to rectify this error, but it still doesn't work (the wheels don't rotate). Why not?
Changing the symbol properties of the item in the Library affects only any new instances you drag from the Library, which will have the properties of the master in the Library. However, instances already in a movie will have the same behavior they started with. Your instances of the Rotating Wheel symbol are behaving like graphics. Go to where they're used (inside the Car symbol) and, with the Properties panel open, select each instance. Then change the Symbol Behavior setting from Graphic to Movie Clip.
I can't find the Loop Once option on the Properties panel when I have a movie clip selected. I swear I've seen it before. Where is it?
You could have seen Loop Once in the Properties panel, but not when a movie clip was selected. Only instances behaving as graphics have this option which is a good reason to use Graphic symbols. Of course, when you become accomplished with ActionScript, you'll find ways to achieve the same effect when using a movie clip although it still might take more work than simply using the option available to instances of graphics.
In this book I keep reading the phrase "instances behaving as graphics." Why don't you just say "Graphic symbols"?
There's a difference. The master symbol has one default symbol behavior, which you can change by clicking the Library's options menu and selecting Properties. However, each instance on the Stage can be changed to something different from the master that spawned it. An instance always starts the same as its master. But not only can each instance be changed (to behave like any type); changing the master has no effect on instances already on the Stage.