Filk songs - what are they?

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I just responded to a discussion where I mentioned filk songs. I checked but didn't find any references to filks on mylot so I thought I'd enlighten people. The word "filk" started as a typo at some long ago science fiction convention where someone wanted to put into the program "science fiction folk songs" but typoed "filk" for "folk." So what exactly is a "filk." A lot of the time its a common tune with new words of a science fiction or fantasy bent. Sometimes its a whole new piece of music. Once in a great while a famous group will write a hit that could be considered a filk by those in the know. The Fixx wrote a song called "Saved by Zero" that upon inquiry they admit was writen because of Doctor Who and the regeneration between Tom Baker and Peter Davison's doctors and the use of/invention of the Zero Room. Toward the end of the song, instead of "we must win" the line is "Who must win."