In theory, they're held in trust by the government. But it's not as if your money sits there in the Social Security trust fund waiting for you to retire. After current beneficiaries are paid, surplus dollars are used to buy bonds from the U.S. Treasury. So the trust has the bonds, but the money is now in the Treasury, where Congress can use it for any purpose.
"The Social Security trust fund is ... a piggybank holding paper IOUs from Congress," Stumpf says.
This is the first year that Social Security has had to cash in one of those bonds in order to meet its payroll, says Stumpf.
"From this point forward, an increasing number of those bonds will have to be pulled out every year -- and Congress is going to have to find a way to come up with all that money," he says.