Making a mash-up is easy in the digital age. You take the vocals-only version of one pop song (this is usually available on the 12-inch vinyl single) and use a computer to splice it onto the instrumental track of another (also available on a 12-inch). The more unlikely the combination, the better. Cheap audio software like Acid helps match the tempos and pitches of different songs. With a few drags and clicks, a lone listener can yoke the majestic Sturm und Drang of Nirvana to the giddy come-on of ”Bootylicious,” by Destiny’s Child — and prompt the unfortunate song title ”Smells Like Booty.” The results are better than this sounds. Indeed, the possibilities of the form are intoxicating.

The fad began when artists like 2 Many DJ’s and Freelance Hellraiser scored hybrid club hits in England, the latter with an oddly harmonious marriage of the Strokes (a guitar-rock band) and Aguilera (a pop singer). Mash-ups then spread to America, distributed via MP3 trading sites on the Web. The mash-up’s success can be attributed not only to accessible technology but also to some ineffable moment when teen-diva melisma and grunge-rock angst sounds not just funny together but also terrific. Mash-ups instantly dissolve the barriers between musical worlds — between, say, ”Total Request Live” and the indie record shop.