You can't, not without changing the question a bit. Think about it: Once
the user has accepted that message box (say if you supply an OK button and
the user hits it), A1+B2 still equals 2; do you want the message box to pop
up again?

1) No; you want it to pop up only when A1+B2 BECOMES 2, that is, when one of
those two values are changed so that the sum is 2. For that you want a
macro; a worksheet function can't do it for you.

2) Yes; you want "Correct" to show as long as A1+B2=2. For that you want
not a pop-up box but another cell that says "Correct" when A1+B2 equals 2,
and something else (blank? "Incorrect"?) the rest of the time. For that,
put =IF(A1+B2=2,"Correct","Incorrect") in the other cell.

--- "Karen" wrote:
I need to have a pop up box saying "Correct" if cell A1+B1=2. How can this be
done?