Many years ago I found out something about hamburgers that really grossed me out. You may not know this, so I hope I don’t make you sick, but it turns out hamburgers are actually made out of dead cows. I am not making this up. Needless to say, as soon as I discovered that, I gave up meat entirely.
I cut my teeth playing rock songs on the accordion when I was a teenager and my friends always thought that was extremely amusing. I think that was the genesis of my polka medleys, because every rock song I played on the accordion just sounded like a polka and my friends thought it was funny. So that was a joke that I continue up to this very day.
So that's why one of my rules of parody writing is that it's gotta be funny regardless of whether you know the source material. It has to work on its own merit.
I do original songs in the style of other artists, where I try to learn all their musical idiosyncrasies and try to do something that sounds like them and yet is a bit more sick and twisted.