3. Select swimming_dog.mp4 in the Use Dimensions From menu, and then click OK to create
the new composition.The footage items appear as layers in the Timeline panel, and After Effects displays the
composition, named swimming_dog, in the Composition panel.
When you add a footage item to a composition, that footage becomes the source for a new layer.
A composition can have any number of layers, and you can also include a composition as a layer
in another composition, which is called nesting.
Some of the assets are longer than others, but you want them all to continue only as long as the
swimming dog is on the screen. You’ll change the length of the entire composition to 7:00 to
match the dog clip.
4. Choose Composition > Composition Settings.
5. In the Composition Settings dialog box, rename the composition movement, type 7:00 for
the Duration, and then click OK.