Social Security is a key part of your financial security in retirement, but many people don't realize that they're not even eligible to receive Social Security benefits. One little-known rule often gets ignored until it's too late for unsuspecting retirees.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, talks about this trap for the unwary. Dan notes that to qualify for retirement benefits, you have to have earned 40 Social Security credits over the course of your lifetime. Dan points out that it's not hard to earn the credits, with earnings of just $1,200 producing one credit in 2014. But because you can only earn a maximum of four credits a year, the requirement essentially leaves those without 10 years of work history unable to claim benefits based on their own work history. As a result, those who start their careers late need to be mindful of the rule in order to avoid mistakenly believing that they've already earned Social Security benefits.

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