"In the short run, the market is a voting machine, but in the long run, the market is a weighing machine." -- Benjamin Graham
Reading Graham's quote today, it's apparent the populace is voting overwhelmingly in favor of a continued market rally, but the real question to ask is, "When will the market weighing machine vote 'no' and send shares tumbling?" To attempt an answer to this age-old dilemma, we'll look at margin account debt balances, which just recently hit a decade record high in April. In the following video, Fool analyst Blake Bos explains what margin debt is, whether these debt balances can predict a sell-off, and how he's placing his investment bets in today's heated market.