Having a pet means teaching it boundaries. For instance, it’s not allowed on your bed (not like it matters to them), the couch (even more so), and it should only drink out of a water bowl. However, after dental problems, this person’s cat couldn’t drink from a bowl, so the owner gave it a normal glass instead.
This would be a mistake. From then on, even after its tooth ache had long gone away, the cat would only drink from cups and glasses. They even set up a permanent glass for the cat in their bedroom for whenever it was parched.