How to let the user edit a rich text box in windows phone?

I've the following xaml code:

<RichTextBox Foreground="black" Background="White"><Paragraph> line 1: abc cde fgh ijk</Paragraph><Paragraph> line 2: abc cde fgh ijk</Paragraph></RichTextBox>

Now how do i let the user edit this RichTextBox like a normal TextBox? (i.e the user clicks on the textbox, keyboard appears and the user can type as usual)