Imagine this scenario: Tom, a young man in England c. 1825, goes to market day in a village in the next valley. While he is there he hears a new song and sings it along with his drinking buddies a couple of times. Walking home that night slightly tipsy he sings the song to himself a couple more times. Not being anything of a musician, he doesn't get the tune exactly right, but it's still a cool tune. Being slightly tipsy, he forgets a few of the words but finds good substitutes.

The next day he sings the song for his mother and sisters. They all like it as well. His sister is a really good singer. She often gets asked to sing at weddings, and has developed her skill to a fairly high degree. She makes the song her own, introducing ornamentation ideas of her own and changing two of the notes to ones that fit her singing style better. In time she teaches it to her daughter, who loves to sing and has a true voice, but isn't much of an artist or musical innovator.

The daughter teaches it to her daughter, pretty much the same way she got it from her mother. The granddaughter also loves to sing but doesn't have the ear or voice of her mother, but she remembers the song as best she can. When she is an old woman watching the world change around her, someone shows up with a tape recorder because he heard she knows a bunch of the old songs. "Ah yes," she says, "I can remember hearing my grandma singing this when I was a baby. My mother sang it often as well," never realizing that the version she is recording is quite a bit different than the one she learned when she was little. The song does, however, retain most of the hallmarks of traditional singing in her area, many of which were introduced by her grandmother. It also retains the word substitutions and slight melodic changes made by her tipsy great-granduncle.

In due time the song gets written down in a folklore book, and people who are researching song origins find the old recordings and write papers about the singing style and repertoire of that area.

Meanwhile, back to tipsy Tom's drinking buddies, specifically the one who taught him the song. He had been a couple of valleys in the other direction for a few days and had learned it there. The person he learned it from had learned it imperfectly from his father, who wrote it. The son passed it down to his nephew, who was really more of a fiddler, but who also liked a good song from time to time. This fiddling nephew sang it a few times at parties, and a young man who lived nearby and was a promising musician learned it and had it running around in the back of his head for a couple of years. Finally, it came out again but with several slight changes that made it fit more securely into the current style of singing in that area -- a style that was somewhat different than what was prevalent two generations before when the song written. This same young man, when he became old, was "discovered" by revival folkies, who put him in a recording studio and sent him on tour.

Back again to Tipsy Tom's song-bearing drinking partner. Remember that he is in a town somewhere between the two towns where the song has been collected so far. He's not much of a singer in an artistic sense, but he's in key and has an ear that can reproduce what it hears. His daughter has the same knack, and when the song is collected from her when she is very old, it is, remarkably, almost identical to the way it was when it was first written.

Fast forward a couple of generations more. People now watch TV and listen to CDs for entertainment, instead of singing and dancing as a community as much as they used to. People who like the old songs have to go out and find them instead of having them sung around them since they were young. They have to form clubs and have concerts in order to hear a larger selection of the music they love. The upside is that they have the internet at their fingertips and can get at least some access to any music they like. The ones who are musicians learn the old songs, but, since so much song sharing takes place in situations that are performances rather than normal life, they introduce arrangement ideas and dynamics that make the song more accessible to an audience that grew up on TV ads. Since they grew up listening to pop music, they are comfortable with playing in bands and generally presenting the music in a way consistent with the current state of music making, which isn't anything like how it was first presented. Some of them even play rock or jazz versions of the songs, or combine them digeridus, gamelan, or what have you. Some folks, on the other hand, really like the old ways of doing things, and they learn the song as closely as they can to the way it was first collected. And people do the songs everywhere in between.

The song remains the same, except that it's been changed by almost everyone who has sung it.