In law this is probably theft. You're dishonestly appropriating something, with the intent of permanently depriving the owner of it. You'll claim that you're

In law this is probably theft. You're dishonestly appropriating something, with the intent of permanently depriving the owner of it. You'll claim that you're replacing it with an item of similar value, but that claim fails because i) the act of replacement means you know you shouldn't just take the cup and ii) the replacement is not equivalent, or you'd just keep the replacement without taking the original. > I'm really not sure if it belongs to anyone. If you knew it was just a generic cup and you were replacing it with something that was equally usable, and the original was not going to be missed by anyone, I'd say it's ethically ok to take the cup. But you don't know this. Maybe that cup belongs to someone and they're just too shy / non-confrontational to tell you to stop using their cup.