Не могу заставить работать gridсплинтер

xaml:

<TextBox HorizontalAlignment="Left" Margin="179,52,0,0" Text="TextBox" TextWrapping="Wrap" VerticalAlignment="Top" Width="66" Height="397"/>

        <TextBox HorizontalAlignment="Left" Margin="245,52,0,0" Text="TextBox" TextWrapping="Wrap" VerticalAlignment="Top" Width="79" Height="397"/>


        <GridSplitter HorizontalAlignment="Left" Height="100" Margin="240,0,0,129" VerticalAlignment="Bottom" Width="5"/>


    </Grid>
</Window>