On change in Record Selection in Sub Form Event

Hi Team,

I want to provide the user with a pop up message box, each time the record selection in the Sub Form of a main form changes.  The selection change can be done using Mouse Click or the Arrow Keys of the keyboard or even by appling the Filter to the Sub form.

I want to get hold of the single even that will help me run my code every time the user changes the record he has selected in the Sub Form.

Thanks,
Sandesh.