When Grandpa meets Lyle, he jokes, "The only physics that I ever took were stewed prunes." "Physic" is an old-fashioned, rarely-used term for "laxative."
Ralph Waite did not appear in this episode due to a contract dispute with CBS. His character's absence was explained by having John Walton away from home working in Norfolk. Yet Ralph's picture comes in the opening credits at the introduction of the show.