When you create a frame-by-frame animation, it's up to you to create every single keyframe and frame. And in cases where you want absolute control over every single image that appears in your finished animation, frame-by-frame animation is the way to go.
Often, however, you can get by with a little less control. If you want to create a scene of a ball rolling across a lawn, for example, you can create one keyframe showing a ball on the left side of the lawn, another keyframe showing the same ball on the right side of the same lawn, and tell Flash to create a tween, or all the keyframes in between. Bingo—scene done.
You can combine frame-by-frame animation with tweening. In fact, that's what a lot of professional animators do: Take care of the complex stuff themselves, and rely on Flash to fill in the spots that aren't as critical.
Tweening saves you more than just time and effort; when you go to publish your animation, it also saves you file size. That's because Flash doesn't save every single frame of a tweened animation the way it does with a frame-by-frame animation. Instead, for tweened animations, Flash saves only the keyframes you create, plus the information it needs to generate the tweened frames from your keyframes. And smaller file sizes are a good thing—especially if you're planning to put your finished animation up on a Web site. (You can find out more about file sizes, including tips for optimization, in Chapter 14.)