宏，当单元格改变值时提醒我-弹出警告- EXCEL

I am looking to create a macro to alert me when a cell's value changes. The cells in this particular column can have values of either "OVER" or "UNDER". I would like to write code to alert me via a popup (Message: "Cell A crosses under(over)") when the value changes.I am looking to create a macro to alert me when