When you're investing in stocks on a brokerage, you may hear about something called "margin." This is basically money the broker will lend you, with your stocks as collateral for the loan. The thing about margin loans is you're only on hook for the loan itself and the daily interest it collects. Anything else is yours.
For example: say you use margin to buy $10,000 worth of stocks. The value doubles after some time. You sell. After paying back the original debt and the interest (which for our purposes we'll say is $2,000), you walk away with $8,000, free and clear.
However, this is incredibly risky during a bear market. If the value of your collateral and/or your loan drops below a certain point, you will experience a margin call: the broker will warn you to either pump more funds in or start selling. Ignore the margin call, and the broker will liquidate the stocks you bought on margin, as well as whatever stocks they have to in order to make back the original loan amount, plus interest.
This doesn't have to be for price action. Using margin to buy dividend-paying stocks before the ex-date can help you execute a dividend capture strategy. Again, though, interest is the killer.
If you have nerves of steel or insider connections, this could be a good way to make money off a loan. Don't be retarded, and confirm with your broker how their margin works.