loop to stop and wait for cell value change.

I'm very new to this, but I have a simple macro that is working largely as intended. I want it to look at a cell, and give the user an error message if the cell value is less than or equal to a referenced cell. I want it to keep checking the cell, but I need to give the user an opportunity to change the value before each check. Can I set up some trigger that activates when the user enters data into the cell?

This is my code:

Sub proControlCheck()
    Range("B2").Select
    Do Until Selection.Value > "A2"
        If Selection.Value <= Range("A2") Then
        MsgBox "The High Control Value is less than the Low Control."
        Range("B2").Select
        End If
    Loop
End Sub

I appreciate any help.