A man walked into a bar and asked the barman for a glass of water. They had never met before. The barman pulled a gun from under the counter and pointed it at the man. The man said "Thank you" and walked out. Why should that be so?

HINTS:
Q: Was the barman expecting some kind of message or messenger?
A: No.

Q: Was the man expecting the barman to pull a gun?
A: No.

Q: When the man said "Thank you," did he mean that he was grateful?
A: Yes.

Q: Was the barman normal?
A: Yes.

Q: Was the man normal?
A: No. He had an ailment

Whoever can answer this will recieve however many mod points I have at the moment (like one or two, sorry). Have fun, PM me for answer but PLEASE do not say it if you've asked me.