Cell Data Validation

[one_more_cave_dweller]

I would like to require the user to enter a number into a2 before they can enter a number in a1. How can I do this

Thanks ray:

 

[ken2step]

Good point Todd! Thanks for catching that one!

 

[vconfused]

But I guess the problem with Data Validation is that you could enter a value in A2, enter a value in A1, then delete the value in A2 without error. The VBA code provided still has the same problem, but could be modified to not have the same flaw. Right?