I am new to Excel VBA can someone please help.

I am trying to write a VBA Excel function that will cause Excel to
say "Fail" if the value entered in a cell is out of the required range.

If as a result of a calculation a cell's value changes. How can I get
Excel to act on that change.

Example:
Enter a value in Cell say B35 that is too high or too low
If B35 < 2.5 or > 3.5 Then say "Fail".