The pentatonic (five-tone) scale is found in folk music around the world as well as in composed music. Almost anything you play in the pentatonic scale sounds melodious because there's no way to make a dissonant leap: it contains no augmented fourth or diminished fifth. The typical major pentatonic scale is built in this pattern: whole step, whole step, minor third, whole step , minor third -- just like the black keys of the piano, or like a major scale without the fourth and seventh degrees. If you play a melody using only black keys you are playing in a pentatonic scale.
"Amazing Grace" is a good example of a major pentatonic melody: its only pitches are F, G, A, C, and D.
"Come All You Fair and Tender Maidens" is a different sort of pentatonic melody. Though it uses the same five notes as "Amazing Grace" it gives a minor effect by emphasizing the "D" and the minor third above it:
There are several possible forms of the pentatonic scale. Below are the major pentatonic, as found in "Amazing Grace," the minor, as found in "Come All You Fair and Tender Maidens," and an alternate form of the minor pentatonic.