Pop-up/hyperlink in a cell

[Guest]

We have a spreadsheet when you roll on that cell a pop-up comes that has a
letter in it. The person can't remember how they did it and it's not
hyperlinked from a word document and we can't see what is in the forumla bar
but it's highlighted like it's a hyperlink.

 

[Dave Peterson]

Data|Validation (look at the Input message tab)

or

Insert|Comment (I'm not betting on this one)