No lineheight?

Hello All,

I set zero to lineheight peoperty of textblock as following.

<UserControl x:Class="SilverlightApplication1.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Width="400" Height="300" Background="Black">

    <Grid x:Name="LayoutRoot" Background="Yellow" Width="400" Height="300">
        <TextBlock x:Name="ShowText" Width="400" Height="300" TextWrapping="Wrap" Canvas.Left="0" Canvas.Top="0" LineHeight="0">
        Take Robert Torrey of Connecticut, the state tied with Alaska as the second most expensive for gas. After l ...