Relative motion at its best - rotate the screw, or rotate the casing, it's all the same. Deceptively simple, like all great inventions!,(reply>), @Memento Mori to my understanding the issue is getting the screw to scoop stuff enough, with the casing rotating the scoops at the bottom grab more stuff to move and therefore can move stuff more easily,(reply>), The question I have is:<br>What's the difference? What does rotating the casing do that rotating the screw doesn't? Why couldn't they just rotate the screw? That simply not covered in this video.<br>I mean yes, it's neat that you can get the same effect by just rotating the casing... but what is the point of doing that? What advantage does it have or what does it do different in comparison?,(reply>), I think it still needs the scoops at the bottom, though, no? If it were JUST a casing rotating, then yes, the screw is "rotating" if you look at it from the perspective of the casing, but so is the material at the bottom, rotating away from the screw. You need the scoops to force the material to spin with the casing and get it going up the screw.