text: The Tween class enables you to move, resize, and fade movie clips easily on the Stage. The following code is from the
  Tween.as class file.
  /* constructor for Tween class obj: reference - the object which the Tween targets prop: string - name of the property (in obj) that will be affected begin: number - the starting value of prop duration: number - the length of time of the motion; set to infinity if negative or omitted useSeconds: boolean - a flag specifying whether to use seconds instead of frames */ function Tween (obj, prop, func, begin, finish, duration, useSeconds) { /* omitted to save space. */ }
  For example, imagine that you want to move a movie clip across the Stage. You can add keyframes to the Timeline and insert a motion or shape tween between them, or you can write some code in an
  onEnterFrame event handler, or you could use the
  setInterval() function to call a function at periodic intervals. If you use the Tween class, you have yet another option where you can modify a movie clip's
  _x and
  _y properties. You can also add the easing methods demonstrated earlier. In order to take advantage of the Tween class, you can use the following ActionScript:
  new mx.transitions.Tween(ball_mc, "_x", mx.transitions.easing.Elastic.easeOut, 0, 300, 3, true);
  The previous ActionScript snippet pixels on the x-axis before animating back using a fluid motion effect.
  If you use the Tween class in more than one place in your Flash document, you might opt to use the
  import keyword. This enables you to import the Tween class and easing methods rather than give the fully qualified class names each time you use them, as shown in the following example:
  import mx.transitions.Tween; import mx.transitions.easing.*; new Tween(ball_mc, "_x", Elastic.easeOut, 0, 300, 3, true);
  Here you use two separate import statements. The first statement imports the
  mx.transitions.Tween class only, and the second
  import statement uses the wildcard (*) shortcut to import each of the six easing classes using a single line of code. The second statement imports an entire package of classes. Flash documentation defines package as "directories that contain one or more class files and reside in a designated classpath directory." In this case, the package resides in the <Flash Install directory>\<language>\First Run\Classes\mx\transitions\easing folder. You might agree that importing an entire package is much better than having to import the six classes separately. Instead of having to refer to
  mx.transitions.Tween, your ActionScript directly refers to the Tween class. Likewise, instead of using the fully qualified class name for the easing classes (
  mx.transitions.easing.Elastic.easeOut), you simply type
  Elastic.easeOut.
  Using similar code, you set the
  _alpha property to fade instances in and out, instead of
  _x, as shown in the following code snippet:
  import mx.transitions.Tween; import mx.transitions.easing.*; new Tween(ball_mc, "_alpha", Strong.easeIn, 100, 0, 3, true);
  Instead of moving around the Stage, now
  ball_mc fades from 100 percent visible to completely transparent in three seconds flat. If you want the symbol to fade out quicker, then you can change the
  duration parameter from 3 to 1 or 2. in seconds. Or, if you set the parameter to
  false, the duration is the number of frames you want to use for the tween. For example, examine the following code:
  import mx.transitions.Tween; import mx.transitions.easing.*; new Tween(ball_mc, "_alpha", Strong.easeIn, 100, 0, 24, false);
  The previous snippet Tween class has a few more cool tricks that you can use. For example, you can write an event handler that triggers when the animation completes, as seen in the following snippet:
  import mx.transitions.Tween; import mx.transitions.easing.*; var tween_handler:Object = new Tween(ball_mc, "_alpha", Strong.easeIn, 100, 0, 3, true); tween_handler.onMotionFinished = function() { //place your action here trace("onMotionFinished triggered"); };
  If you test this ActionScript in your FLA file, you see the message "onMotionFinished triggered" appear in the Output panel after
  ball_mc finishes fading on the Stage.