What would you do if you witnessed this?
A young girl is bullied in a park by two other girls - but it's all a social experiment.
The youngsters are acting out the roles of bullies and victim, unbeknown to the strangers who witness the encounter.
How do they react? What do they say or do? See what happens in our video (above) - and also discover when the strangers react in the way they do.
Meanwhile, Sir Paul McCartney is among the stars to appear in an anti-bullying music video (below).