When I was young, I read popular physics books such as Richard Feynman’s QED:
The Strange Theory of Light and Matter. I knew that light was waves, sound was
waves, matter was waves. I took pride in my scientific literacy, when I was nine
years old.

When I was older, and I began to read the Feynman Lectures on Physics, I ran
across a gem called “the wave equation.” I could follow the equation’s
derivation, but, looking back, I couldn’t see its truth at a glance. So I
thought about the wave equation for three days, on and off, until I saw that it
was embarrassingly obvious. And when I finally understood, I realized that the
whole time I had accepted the honest assurance of physicists that light was
waves, sound was waves, matter was waves, I had not had the vaguest idea of what
the word “wave” meant to a physicist.

There is an instinctive tendency to think that if a physicist says “light is
made of waves,” and the teacher says “What is light made of?” and the student
says “Waves!”, then the student has made…