Heavy Cream

I've been skimming only the very top of the cream, the really thick stuff, the stuff that whips up nice for desserts. The cream jar happens to be full to the brim right now. (Ah, the luxury!!)

A child took a nearly full jar of milk from the fridge and asked about starting a new cream-jar. "Nope," I said, "I already skimmed that one." He looked quizzically at the layer of cream floating on the milk. I continued, "I've only been skimming the heavy cream. You know, the stuff that sits at the very top that's all thick."

Sits at the top of the jar? Didn't we learn in science class that the stuff on top is the lightest, the least dense? What's heavy goes to the bottom of the jar. So what's with the term heavy cream?