These are heady and concerning days for those of us who follow financial markets. Irrational exuberance is everywhere. Stocks are trading at or near their all-time highs and bonds are moving upwards in defiance of anything that resembles common sense. As a journalist and former longtime M&A banker, I have always enjoyed rolling calls—I dial myself of course—with some of the keenest minds in the financial industry, often to jawbone about the absurdity that occasionally overtakes the peculiar business of making money from money. More recently, however, my conversations with Wall Street executives and leading economic thinkers have turned to the darker, inevitable question of what happens when the music stops, as it surely will, and soon.