A mask is something that hides one part of an object and exposes another part of the object. Creating a mask is simple:
Place the object to be masked on one layer, and the object that will act as a mask on the layer just above it.
The mask can be a path, a Drawing Object, a group, an instance of a symbol, a block of text, or just about anything else. Its stroke and fill colors don't matter (Figure 41a).
Figure 41a Top, a mask (the text) and an object to be masked. Below, the same objects after the Mask command has been applied.
Right-click (Windows) or Control-click (Mac) the name of the layer containing the mask, and select Mask from the contextual menu.
Flash converts the upper layer into a mask layer and the lower layer into a masked layer. The object on the Stage appears masked.
A mask on its own is no big deal. What makes masks interesting is that you can animate the mask, the masked object, or both, thereby achieving complex effects quickly (Figure 41b).
Figure 41b The bubbles in the water are shape-tweened paths; the laboratory flask is a motion-tweened instance of a symbol. The animated flask is used to mask the animated water, allowing the water level to stay the same no matter which way the flask is tilted.
It's possible to have one mask layer affect multiple masked layers. If there's a layer that you want to put under the control of an existing mask, drag and drop it between a mask layer and a masked layer in the Timeline (Figure 41c). The masked layers can be arranged in any order, so long as the mask layer is above them all.
Figure 41c In this Timeline, one mask layer is affecting two masked layers.