Take a look at the Validation entry in XL Help ("About setting
restrictions on cells and displaying input and error messages")

In article ,
George wrote:

I am using the spreadsheet as a tracking mechanism, how do I format a cell so
it will only take a certain entry. For example, the formula is set to only
count s1 or s2 or s4. If someone hits s5 one data entry, I want the cell to
reject it.
Thanks