Rövarspråket
A Swedish children's word game where every consonant is doubled with an o in between.
The rule is simple: each consonant becomes the consonant, an o, and the consonant again, while vowels stay as they are. "Hej" becomes hohejoj. It is not cryptography in any serious sense, but the mechanism is a substitution with one fixed, publicly known rule.
Kerckhoffs's principle applies here too. Security does not rest on keeping the method secret, and anyone who knows the rule reads the text at once. That is precisely why it is a language game and not a cipher.