changing tooltip based on state in silverlight

Question

Hi, I wanted to be able to change the tooltip on checked and unchecked through the visualstatemanager, this is what i got so far but it doesn't work:

<VisualState x:Name="Checked">
  <Storyboard>
    <ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetProperty="ToolTipService.ToolTip" Storyboard.TargetName="btn">
      <DiscreteObjectKeyFrame KeyTime="0">
        <DiscreteObjectKeyFrame.Value>
          <System:String>Button is checked</System:String>
        </DiscreteObjectKeyFrame.Value>
      </DiscreteObjectKeyFrame>
    </ObjectAnimationUsingKeyFrames>
  </Storyboard>
</VisualState>

any suggestions or workarounds would be greatly appreciated thanks