UserControl and Two-way Binding

Thanks Bigsby,
The thing is that I have no class of my own.  I want to use a string.  All I want is a user control that contains a textbox.  I want the DataContext of the user control to be assigned a string (actually bind it to a property of any other object).  Then I want to show this string in the inner textbox (keeping the binding).  When I update the string using the inner textbox, the change must get communicated back to the property bound to the DataContext. 
Thanks