title: How to hide an object behind another one?
body: <p>I am trying to make a storyboard for my button with Expression Blend.
  I would like it to move, and then to disapear behind some virtual panel.
  I think I could do it by defining an "area of visibility" for my button, so when it moves out of that area, it is hidden. Is it possible ?
  Thanks for your help.</p>