Mashups can be pretty cool, finding 2 songs that have enough in common like the key they're in, or the rhythm between the 2 that fits to blend them together. Back in the day, we used to use reel-to-reel, a razor blade, a grease pencil, and a splice bar to mash things up. But in this digital age it's a lot easier. The results can be pretty impressive. Check out this mashup of Metallica's Sad But True and Stevie Wonder's Superstition below.