You would have to loop through all the records and set them all to false and the one you click on is true.

For Each Loop

Thanks for the reply. I'm breaking new ground here, so I'm trying to keep up.
This is what I've managed so far and all it will do is make it so that once I mark a pet as favorite, I can't mark it as not favorite. I tried doing the S as String thing from the Status Box video thinking that was a prerequisite for this process. I obviously don't understand the basics of trying to accomplish this loop thingy. Let me know if I'm getting close or if I need to do more research.

Private Sub FavoritePet_Click()
  Dim C As Control
        For Each C In Me.Controls
              If C. ControlType = acCheckBox Then
                     FavoritePet.Value = False
              End If
        Next
End Sub

You basically need to handle two situations:  (1) If an unchecked checkbox is clicked, check it, and uncheck the other checkboxes (pet owner favoriting one pet and unfavoriting the others).  (2) If the user clicks on a checkbox that is already checked, uncheck it (pet owner unfavoriting a previously favorited pet, leaving him with no favorite pet).

The second condition above may not be what you want, so you have to decide what to do if the user unchecks a previously checked checkbox, which will leave you with all boxes unchecked - no favorite.  If you must have at least one Yes, you need to find a way to force the user to check at least one box in that situation.  Or you can allow the user to have no favorite pet.

Use an UPDATE statement in the After Update event for the Check Box.

Private Sub IsFavourite_AfterUpdate()

    CurrentDb.Execute "UPDATE PetT SET IsFavourite=False"
    
End Sub

Add an appropriate WHERE condition, if needed. For example:
    CurrentDb.Execute "UPDATE PetT SET IsFavourite=False WHERE CustomerID=" & cboCustomers

That will ensure only one favourite pet is selected - per customer, if applicable

.Access SQL Seminars - Part 2 covers Action Queries.