This is not a joke question. I've learned something strange through the reply to a minor interpellation by parliament.
One would assume that a passport is a proof of citizenship of that country? In Germany apparently it is less than proof. In Germany a German passport only counts as presumption of German citizenship.
They wrote, in German language, and translated by me to English (German original below):
The citizenship card is the only document with which the existence of German citizenship in all matters for which it is right - is significant and binding (§ 30 StAG). The German passport and identity card are no proof for the German Citizenship, they justify a presumption that the holder of the German nationality.
Most Germans poses a German ID card and a lot posses a German passport. The citizenship card (Staatsangehörigkeitsausweis) however is a rarity of which in the year 2018 only a number of 174558 existed (Germany has a population of 83 million). This number comes was soured from another minor interpellation by parliament. Rather, noways it is difficult for Germans born citizen to acquire a citizenship card (Staatsangehörigkeitsausweis). Authorities refuse to process applications for citizenship cards due to "lack of justification" (German, really hard to translate, "fehlendes schutzwürdigen Sachbescheidungsinteresse"). Even a court ruled:
Applications for the determination of the existence of the German nationality (§ 30 paragraph 1 sentence 1 StAG) are rejected if they are lacking the necessary dignity of interest.
Let me summarize that for you:
The German passport and identity card are no proof for the German Citizenship, they justify a presumption that the holder of the German nationality, yet authorities refuse to process citizenship cards, which would be proof, and a court agreed with them. In the view of German administration, unless German citizen specially are required to have a citizenship card for some purpose, they don't have the right to acquire a document that would proof that they are German citizen.
Is a passport proof or presumption of citizenship?
Der Staatsangehörigkeitsausweis ist das einzige Dokument, mit dem das Bestehen der deutschen Staatsangehörigkeit in allen Angelegenheiten, für die es rechts - erheblich ist, verbindlich feststellt wird (§ 30 StAG). Der deutsche Reisepass und Personalausweis sind kein Nachweis für die deutsche Staatsangehörigkeit, sie begründen nur eine Vermutung, dass der Inhaber die deutsche Staatsangehörigkeit besitzt.
Landtag von Baden-Württemberg 16. Wahlperiode Drucksache 16/1883 04.04.2017 https://www.landtag-bw.de/files/live/sites/LTBW/files/dokumente/WP16/Drucksachen/1000/16_1883_D.pdf