如何“提醒”用户填写工作表上的单元格

Hello: I have a set of cells on a worksheet called "Docs". The cell range is (B13:C23). When users get taken to this page, they are meant to fill out each of these cells with a value from 0 through 6. My Question: Is there some code that I can attach to this sheet where, if a user does not fill in a cell with anything (ie. leaves it blank) and tries to leave the sheet or close the workbook, they are somehow reminded to fill it in? Or is there a way to not let them leave the sheet until it's completed? Thanks.. AllanHello: I have a set of cells on a worksheet cal