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

$('comments').addEvents({
  focus: function(){
    myTween.start('height', 200);
  },
  blur: function(){
    myTween.start('height', 50)
  }
});

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

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