Allright, let me try to explain it this way. You're in your car on wet, perfectly level ice. You try to accelerate, but your wheels just spin and you go nowhere (for the sake of argument). Try as you might, as your tires push against the ice, they get no friction and can't get you moving. If, however, you applied an external force to the car, like a big-ass fan, then the car would move over the ice. Why is the car moving now? I mean, isn't it impossible to apply torque to the wheels to move the car? It's moving because it's NOT pushing against the ice, it's pushing against the air.

Does that convert anybody? I doubt it.

::edited for clarity::