WPF Binding妙处-既无Path也无Source

<Window x:Class="XamlTest.Window12"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        Title="Window12" Height="300" Width="300">
    
    <Grid>
        <Grid.DataContext>
            <sys:String>Hello</sys:String>
        </Grid.DataContext>
        <StackPanel>
            <TextBlock Text="{Binding}"></TextBlock>
        </StackPanel>
        
    </Grid>
</Window>