Financial Physics rule five asks the uncomfortable question every investor should answer: what is the worst that could happen? Don and Tom revisit leverage in 1929, the crashes of 2000, 2008, and 2020, and the practical defenses that keep a bad market from becoming a ruined plan. Then the questions turn to retirement planning: managing IRMAA while considering Roth conversions, weighing long-term-care insurance against self-insuring, and judging whether a $1.6 million portfolio can support a ...