Boarding the train to DC this morning I noticed again that people take their seats the way electrons fill an atom’s orbital “shells.” Each person grabs a seat in an empty two-seater rather than sit next to another person in a half-filed row. (All rows are two-seaters.) And in an atom each electron will first fill an empty orbital before pairing in a half-filled orbital with another electron. (All orbitals are two-seaters.) Really. The observation was German physicist Friedrich Hund’s, back in Einstein’s day, only he didn’t ride Amtrak. He rode busses.