The pentatonic scale is composed of only five notes, as opposed to the tradition seven-note scale. Imagine playing a song using only the black keys of a piano. As difficult or unnatural as this may seem, plenty of very common songs are composed using a pentatonic scale, including "Cowboys" by John Williams and "My Girl" by The Temptations. In fact, one of the first original songs that I wrote for marching band turned out to be in a pentatonic scale. I didn't even realize it until years later!
I found out about Bobby McFerrin a while ago through YouTube and I'm amazed by everything he does. A coworker recently sent me a video to a demonstration he gave at a conference called "Notes and Neurons." It's an interesting look at how the pentatonic scale is almost hardwired into the human brain. Check it out!
