Something prompted me to read "Physics," published in 1960, by
Physical Science Study Committee. In Chapter 1 I see the following.
1) Take another chain of events. " ... The solid bar [tested on a
stretching machine] slowly yields to the pull. It stretches out like a
piece of taffy, and breaks a little later with a jungle of sound. How
does it all work? What holds the bar together? Why did it fimally give
away? And, by the way, why is steel stronger than glass? Why is it
more dense than aluminum? And why it rusts? The words "how" and
"why," which I colored, remind me a recent suggestion that the word
"why" should not be used by physicist.
2) How to answer the "What hold the bar together," question? It begins
with the word "what."