Quick German: Do you know the meaning of "Berliner"?
Actually there are two meanings:
(1) A man from Berlin, the German capital (A woman from Berlin is a "Berlinerin")
(2) A Berliner Pfannkuchen (in short called Berliner) is a traditional North German pastry similar to a doughnut (without a central hole) made from sweet yeast dough...
John F. Kennedy declared in his famous speech in Berlin on June 26, 1963:
"Ich bin ein Berliner!"
It caused German speakers (especially young teenagers, like me!) to chuckle slightly - not because they did not understand the point Kennedy was making, but because in German indefinite articles are not used in front of a person's place of origin/birth, nationality, profession, occupation, etc., e.g. "ich bin Hamburgerin"; "ich bin Deutscher"; "ich bin Arzt"; "ich bin Studentin." (Note, however, that you do use "ein/eine", when you add an adjective, e.g. "ich bin ein guter Arzt".)
(You might also chuckle, if a German were to tell you: "I'm a Hamburger", meaning that he is from Hamburg.)
Someone reminded us, however, that we should point out the following: In the context of Kennedy's speech, the use of "ein" was quite appropriate as he did not mean to define his origin, but rather his being "Berliner-like", i.e. he used "Berliner" as a predicate adjective, as in "Ich bin ein Berliner Mann".
Let us know any comments or questions you have and - keep learning.