Problem: Customers ask for a cup for water in restaurants with selve serve soda fountains and proceed to fill the cup with soda. (Is this a big problem?)
Solution:
QR codes (or something else) printed on the inside bottom of each cup intended for soda. (water cups would be normal cups.) Then the machine would only dispense soda if it sees the QR code. (Or same idea with RFID)
The QR codes could also be used to gather data about your customers - what's their favorite drink? How much do they drink per visit? What size drink do they prefer?
I think you could get this data by tracking (1) How many people came in, potentially via number of meals sold (2) How many times you have to re-order each type of drink. It wouldn't be real time but the data presumably already exists. Plus nowadays I see those fancy electronic dispensers with touch screens and would be very surprised if they didn't have built in metrics.
I think this is interesting, but I suspect any solution will cost more than the loss incurred from the problem. But, ignoring that,
A good way to implement it might be to have an entire line of cups that have the QR code printed, with a peel-off sticker inside hiding it. Then you could just have one type of cup and peel it off whenever someone asks for water/soda.
Solution: QR codes (or something else) printed on the inside bottom of each cup intended for soda. (water cups would be normal cups.) Then the machine would only dispense soda if it sees the QR code. (Or same idea with RFID)