Step 2:

Drag your mouse over the movie and click on it. This will enable you to type in the desired text.

Step 3:

Type in the desired text.

Step 4:

Now select the Arrow tool and click on the text.

Step 5:

Go To: Insert > New Symbol, or you can just press F8.

Step 6:

Now name the symbol whatever you want, remember, motion tweening requires the objects to be symbols.

Step 7:

In the timeline, make two other keyframes from your first one, ten instances apart.

Step 8:

Select the arrow tool, then select the scaling tool, then re-size the text to the desired size.

Step 9:

Select the firs keyframe, and go to Insert > Create Motion Tween, do that for the second keyframe also.

Step 10:

Select the last keyframe and go to the actions menu, select the following actions: "Go to and Play (1)". This will make the movie continuos without taking any breaks.

You should end up with a timeline similar to this one:

To test your movie, hold down the keys, Ctrl + Enter.

Demo: