Trump's street-brawling style delivers a knock-out punch to political debate in US
After watching 'The Karate Kid' too often as a child, I decided to take taekwondo classes when I was about 12. During the sparring sessions, I quickly learnt that the most dangerous person to fight was not the 6ft 3in black belt, but the guy who walked in straight off the street.
A fighter with a black belt might have superior technique, but he also played by a set of predictable rules. Much more dangerous was the "newbie", who was unorthodox and unpredictable.
I was reminded of this recently as I watched Donald Trump race ahead of other Republican US presidential candidates by breaking all the rules.