Perhaps Keith was referring to p. 50?

> "The basic idea in manufacturing is exactly the same as that for chemistry, except
there is an important assumption we can make in manufacturing that does not hold
for chemical reactions:

> You can throw anything you want away, and it disappears from view.

> This simple assumption has caused the world some significant problems, but it is still
in effect. In our meringue pie example, we can ask: “what happened to the egg shell,
or the paper surrounding the stick of butter”? The answer is they were thrown away"

I just learned that, apparently, pressing Tab posts the comment!