Millions of us in America play fantasy football, but the way you play could be against the law.

It's estimated that 20 million Americans are part of a fantasy football league this season. The federal government doesn't have a problem with cash prizes for the winners, as long as the prizes are not based on the number of players or the fees they pay. Forbes also reports that it's against the law when there's more chance involved than skill.

State law also comes into play, and generally speaking, most states fall in line with federal law, but some are tougher on the game of chance part, so player beware.