Filling a Rectangle at different levels with Silverlight Animation?

I am very new to Silverlight. I have a question on Animation. My requirement is, a rectangle should have different levels of fill indication. More clearly say, my 开发者_C百科rectangle is half filled initially, it has to be either increased/decreased with animation. Can anyone help me out how can I do this in Silverlight animation?

draw a 2nd rectangle inside the 1st rectangle. adjust the height of the 2nd rectangle to fill up/down.

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