Quodlibet!

I'll bet if you are a pianist, you can play a quodlibet right now - or maybe one of your brothers or sisters knows this one. Do you know "Heart and Soul"? How about the second part, you know, the one that goes, "Dat, dat, daaah, Da-da, da-da, da-da..." That's a quodlibet!

The most famous musical quodlibet can be found in the last variation of a set written by Sebastian when he was a grown-up. The entire set of variations was written for the harpsichord and is called the Goldberg Variations.
Variation #30