A very Scandinavian joke.

Swedes and Norwegians (and Icelanders) almost sound like they’re singing when they speak, while Danish is remarkable in that it has no accent at all. It’s very flat, not unlike German. Yet Danes are still somewhat understandable to Swedes and Norwegians, because Swedish, Norwegian and Danish are more or less the same language, so to them it sound like “Danes are speaking with a potato in their mouth”

But now the secret is out. We don't actually talk like that when we're alone. We only do it to annoy Swedes and Norwegians.

And yay! Norwegians making fun of the Danish language [link]
And a Dane making fun of the Norwegian language [link] (Danish without subtitles. Sorry)