Explain the purpose of a mask and how to create a mask.

What will be an ideal response?

For a special transition or special effect that uses the shape of one object to reveal content in another shape, you can create a mask. A mask on one layer exposes content on the layer directly beneath it, but only within the confines of a particular shape. To create this type of animation, you first add content to the masked layer, the layer that will be revealed by the mask. Next, you create a shape on the mask layer, the layer above, through which content will be viewed.