If 20% of people believe that Abortion should be legal for 17 year olds, without parental notification, then you could buy stock in this idea at $0.20 a share. If The percentage of people who believe this goes up, you could sell your shares for a profit. If it goes down you could either keep them, and risk that it goes down further, or you could sell them at a loss.

In order for "The Idea Stock Exchange" to have money to pay people out we would have to take a small transaction fee. Also I imagine you would have to pay $10 or so to participate. You could probably take this money with you when you no longer want to participate. So on average people would loose money, but you would sign people up, like all gamblers, thinking you would be able to beat the system. If you are smarter than 55% of the population, you would gain money.