Make a grid into a grid XAML - xaml

Question

I want to create a grid into a grid. I tried the following piece of code, but this doesn't work, does anyone know why? And Do you have a solution?
<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" ShowGridLines="true">
<Grid.Background>
<LinearGradientBrush>
<GradientStop Color="#EEE8AA" />
<GradientStop Color="#2F4F4F" Offset="1" />
</LinearGradientBrush>
</Grid.Background>
<Grid.RowDefinitions>
<RowDefinition Height="10*" />
<RowDefinition Height="90*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="10*" />
<ColumnDefinition Width="80*" />
<ColumnDefinition Width="10*" />
</Grid.ColumnDefinitions>
<Grid Grid.Row="0" Grid.Column="1">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="100*" />
</Grid.RowDefinitions>
</Grid>
</Grid>

Answer 1

The code is correct. You wont see anything because the second grid just fills a specific grid cell in the parent grid and has no content. Try adding some text boxes or colours and you will see that it works fine.
See the sample below. You will have 3 text boxes displayed in your second grid correctly.
<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" ShowGridLines="true">
<Grid.Background>
<LinearGradientBrush>
<GradientStop Color="#EEE8AA" />
<GradientStop Color="#2F4F4F" Offset="1" />
</LinearGradientBrush>
</Grid.Background>
<Grid.RowDefinitions>
<RowDefinition Height="10*" />
<RowDefinition Height="90*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="10*" />
<ColumnDefinition Width="80*" />
<ColumnDefinition Width="10*" />
</Grid.ColumnDefinitions>
<Grid Grid.Row="0" Grid.Column="1">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="100*" />
</Grid.RowDefinitions>
<TextBlock Grid.Column="0" Grid.Row="0">TextBox1</TextBlock>
<TextBlock Grid.Column="1" Grid.Row="0">TextBox2</TextBlock>
<TextBlock Grid.Column="2" Grid.Row="0">TextBox3</TextBlock>
</Grid>
</Grid>

Answer 2

If you want to see the Grid visually without adding content to it, at least try to set it's Height/Width to a fix value. I copy-pasted your code inside StackPanel set the outer Grid's Height to 100 :
<StackPanel Background="Gray">
<Grid ShowGridLines="true" Height="100" >
.......
</Grid>
</StackPanel>
Result :