"Bust my buffers" - neutrons disentangle the quantum nature of matter

“Because that is the way it is” I finally answered to our son’s favorite word “Why?”. He had discovered – like most kids his age – that his wooden train with magnetic buffers would pull one end of the wagon, but repel the other end. His first “why” was easily rewarded: “because they are magnetic”. But another 3-4 “why”s sliced through our entire physics education: “Magnets have 2 poles (north and south), we call them dipole magnetic moments. The magnet is made of many tiny (atomic) magnetic moments. The atomic magnetic moment comes from …”. Suddenly he had me cornered with only two possible routes of escape: Maxwell’s laws for electromagnetism, which is the consequence of Einstein’s theory of relativity for moving electrons, or Quantum Mechanics, which cause particles to have intrinsic magnetic moments called a ‘spin’.