Edit in JSFiddle

var myTween = new Fx.Tween('comments',{
  transition: 'bounce:out',
  link: 'cancel',
  duration: 1000
});

var grow = function(){
  myTween.options.transition = 'bounce:out';
  myTween.start('height', 200);
};

var shrink = function(){
  myTween.options.transition = 'quad:in:out';
  myTween.start('height', 50);
};

$('comments').addEvents({
  focus: grow,
  blur: shrink
});

<textarea id="comments">Focus and blur me all night long</textarea>

textarea {
    width: 200px;
    height: 50px;
}