That's the root of the puzzle! No matter how hard you try, you can't make *that* happen!

As soon as the plane throttles up, it's gonna move forward. Doesn't matter how hard you pull backwards on that conveyor belt, the plane goes forward at more or less the same speed that it would whether the conveyor belt was there or not.

You can only keep the plane from taking off in two ways: 1. Applying its brakes. 2. *Not* throttling up. But those would keep the plane from taking off whether there was a conveyor belt or not.

Here's another visualization of it that Vixy mentioned to me as we were watching the episode last night: What if we took flight out of the puzzle completely? What if the plane were just taxiing? The puzzle ends up being exactly the same, with the same result. If you throttle the plane up to the place where it would normally taxi at, say, 10mph, and you pull back on that conveyor belt at 10, 20, 30, or even 50mph, the plane will still move forward at 10mph.