Empty Word bag!

Richard Feynman tells us how he tried to explain physics to his father.

He was happy with me, I believe. Once, though, when I came back from MIT - I'd been there a few years - he said to me, "Now", he said, "you've become educated about these things and there's one question I've always had that I've never understood very well and I'd like to ask you, now that you've studied this, to explain it to me," and I asked him what it was. And he said that he understood that when an atom made a transition from one state to another it emits a particle of light called a photon. I said, "That's right."

And he says, "Well, now, is the photon in the atom ahead of time that it comes out, or is there no photon in it so start with?" I says, "There's no photon in, it's just that when the electron makes a transition it comes" and he says "Well, where does it come from then, how does it come out?" So I couldn't say, "The view is that photon numbers aren't conserved, they're just created by the motion of the electron."

I couldn't try to explain to him something like: the sound that I'm making now wasn't in me. It's not like my little boy who when he started to talk, suddenly said that he could no longer say a certain word - the word was "cat" - because his word bag has run out of the word cat. So there's no word bag that you have inside so that you use up the words as they come out, you just make them as they go along, and in the same sense there was no photon bag in an atom and when the photons come out they didn't come from somewhere, but I couldn't do much better.