Shape Tween

Create a new flash file (Ctrl+N). 

Make a short line using the line tool.

On the timeline, click on frame 25 and press F6 to go to it. Once on this frame, make the line longer using the Subselection tool.

Right click on any frame between 1 and 25 and select Create Shape Tween.

Press Ctrl+Enter to view your line get longer.

 After you finish, your object should look like the one below.