<Canvas xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
  <Button>
    OK
    <Button.Triggers>
      <EventTrigger RoutedEvent="Button.Click">
        <EventTrigger.Actions>
          <BeginStoryboard>
            <Storyboard TargetProperty="Width">
              <DoubleAnimation From="50" To="100"
                Duration="0:0:5" AutoReverse="True"/>
            </Storyboard>
          </BeginStoryboard>
        </EventTrigger.Actions>
      </EventTrigger>
    </Button.Triggers>
  </Button>
</Canvas>