Some people exclusively drink soda! Like over 40oz a day of it. It's nasty, but I've seen people carry around those cups and just soda all day.
Their excuse is usually...well soda has water it in, so I'm getting my intake. While technically correct, it's the other trash in soda! Ugh. Americans man.
I considered saying just that, but it didn't seem like a winning situation. I believe I wished her luck finding something she could drink then wandered off until she was checked out.
The conversation had begun because I picked up a then-new flavor of soda and she was asking about it to replace her current soda choice or something like that. Perhaps she thought the carbonation or syrup neutralized whatever she believed she was allergic to in the water. Maybe she was confused by that movie The Others. We may never know and I'm okay with that.