Next time you see the "go ahead" advert, it's a cereal bvar for slimmers I think, watch carefully.
She take a bite out of the top corner as she walks along, then nearly has another bite half way through but at the end when she's sat on the bench the bar has magically become whole again!
If you look carefully you may spot that she has opened the packet at both ends, so when it looks as if she's on the bench with a new bar she is in fact about to start eating it from the other end.