A blonde walks into a library...

A blonde walks into a library and shouts "I'LL HAVE A COKE PLEASE." The librarian says, "This is a Library." The blonde says, "Yes I know. I'LL HAVE A COKE PLEASE." And the librarian says, one more time, "This is a library Miss." The blonde goes "Oh," and whispers, "I'll have a coke please!"

This is a blonde joke. The humor in the joke is the fact that the blonde person misunderstands the reason why the librarian responded to the ordering of a coke with the response of "this is a library". The reason why the librarian said "this is a library" is because you don't order food or beverages at libraries - often libraries forbid food and drink altogether, but the blonde assumes that the librarian is saying "this is a library" because you are not supposed to speak loudly at a library.

In fact, the idea that librarians don't like people to speak loudly is a pretty common trope, so this is a reasonable thing to think if for some reason you would expect libraries to serve beverages.