I knowed there was a trick to it. TV ad about some super kitchen knife made out of what, transplutonium boronitride or something.
So they show the actor rubbing the edge on a brick, then slicing a tomato slick as you please.
Wow ! Wotta demonstration !
Until you notice that the rubbing on the brick is on the blade near to the hilt, and the slicing is done with the undamaged edge nearer the point.
Ha! Gotcha !