I'm no hans schubert, but I think "mary had a little lamb" had tempo that is constantly changed around to make popular songs, hence fun, aesthetically addictive. The odd 16th note there, the odd 8th note there. It's like when you're editing trying to create a mosaic out of a grid of squares; The square is the original shape, but if you pull the sides in different directions then you get the beautiful mosaics that MC Escher makes.