So I have a water bottle in my office and to fill it every time is a tiring business. I need to walk to a different floor than I sit and then I need to wait till other people fill their bottles. Then after filling it, I need to come back to my office.
Here is a colleague of mine who sits next door and comes to my room now and then for discussion. He is a good person and I don't have any other complaint about him. I, in fact, like to discuss with him. But whenever he comes, he would drink almost all the water from my water bottle without asking me! Now, I have two problems with this. First is obvious. As I said already, it is a tiring business to refill the bottle every time. This guy doesn't carry his own bottle and relies on mine (and probably of others') bottle(s). Second, I find it strange that he doesn't ask me whether it is fine to use my water.
I want to convey these two things to him somehow. But it may look too insulting to stop someone from drinking a water(!). What can I do?