Photo © Danilo Alfaro
Making stock is a perfect example of this. Simmering bones not only extracts flavor, it also breaks down the collagens in the cartilage and other connective tissues, adding body to the stock. Once this is done, however, one might feel inclined to chuck those bones in the bin. After all, what good are they?
Plenty, it turns out. Not content to use these bones just once, our thrifty culinary forefathers invented remouillage (literally, "rewetting" in French), which refers to a stock made by resimmering bones that have been used to make stock once already. And why not? It still beats cooking with water. I'm convinced that some medieval chefs would resimmer those bones until they'd completely dissolved — and still not be happy. In fact, I think I worked for one of them once.
Here's some more info about making stock: