Twin brothers Pat and Sean Kelly are musicians and DJs for private events in Philadelphia. When you mix music on the fly, you get a feeling for what works together, and an awful lot of songs work together. You know sometimes you get a song stuck in your head, and it reminds you of another song that uses the same chord progression, and then that song is stuck in there as well. Take this to the limit and you've got magic, especially if you've got a brother to sing with. They call this medley "My Brain at 3:59am – A Mashup." Twenty songs in one minute, divided by two singers, means each song gets about six seconds, but that's long enough for you to know what song it is, and now you've got an entire mashup stuck in your head. If you hear any songs you don't know, there's a list of them at Boing Boing.