I actually started with eels. Aal (=> eel) can be inserted basically everywhere by just stretching the “a” a bit, like “Aalle wissen das” instead of “Alle wissen das” (everyone knows that). As far as I know, this started (or at least became popular) via Jan Böhmermann, he had some jokes about that in his show - the same guy who wrote the “goat fucker”-poem about Erdogan that caused massive diplomatic problems and resulted in changed laws.