For a simple one put yes and no in 2 cells off to the right somewhere,
say J12 and J13.
Then click in A1
Go to Data>Validation>Settings Tab
From the Allow dropdown select List
In the Source box type =$J$12:$J$13
OK out and you are done.
You can enter up to 255 characters in the delimited list.
The items will be case sensitive, so if the list has yes, no as items,
users will not be able to enter Yes or YES.
Also, you won't be able to include items that contain a comma.
I've just been re-reading your website. Obviously, I have
to learn to scroll down more often instead of glossing
over things things I think<g> I know.
By the way I'd like to take this opportunity to say
what a tremendous resource your website is. I often
show it to work colleagues in other sections of the
department. The look of awe and revelation on their
faces is a sight I wish you could see.
Thanks Martin, that made my day! I'm glad to hear that you like the
site, and I'm fortunate that some very talented people have allowed me
to publish their work there.