macro: use selected characters

I want to create a macro that will allow me to highlight characters in a cell, then do the equivalent of:
Format > Font> Superscript

Here is my macro:

Sub superscript()
'
' superscript Macro
' Macro recorded 7/10/2008 by LKB
'
' Keyboard Shortcut: Ctrl+shift+w
'

    With Selection
        .superscript = True
        .Subscript = False
    End With
End Sub

But it doesn't like the With Selection (i get an error). how can I get it to do the operation on the selected characters?
Thanks.