Data Validation

hi all
could someone help me on a data validation exercise
basically I want to control what a user enters into a single cell based on others eg

A B
1 20 500
2 50 500
3 75 500
4 100 300

column A is a thickness and column b is a width what I want to happen if the user enters a thickness in cell D1 and a width in cell D2 I want an error to pop up if for instance the user enters 100 as a thickness to only allow a maximum of 300 in the width. likewise if the user enters a thickness less than 100 in cell D1 to allow him to enter up to 500

I hoe I have explained it ok

thanks

simon

hi
thanks for the reply, however why cant I enter the grid references from another sheet??
the data is in a table on a different sheet

sorry probably should have mentioned that!!

thanks again

Simon

In Excel, the data validation feature helps you control what can be entered in your worksheet.