In roulette, if you place a bet on the color black, you double your bet. The color black comes up slightly less than one time in two (on average). So theoretically, if you kept doubling your bet you would always come out on top, as long as the colour black came up once in a while. Why doesn’t this work in practice? (It doesn’t–I tried it!) Leaving out the possibility that the game was fixed.
I mean, the idea is that if you lose, you double your previous bet and keep doubling it until you win. Then you go back to your initial stake and start again.