You are in a town with one barber shop and two barbers, one with good hair and one with terrible hair, there are no towns nearby so everyone in town must get there hair cut at this barber shop.
Which barber should you choose to cut your hair?
The answer is:
Choose the barber with terrible hair.
The reason is:
The barber with good hair can not cut his own hair and has to rely on the barber with terrible hair. This results in his obviously good. For the other barber, it is the other way round: the barber with good hair is not able to give a good hair cut.
Go ask someone in the city with a good hair cut "From whom they did their hair cut"?
By that you can understand who is a good barber.
Some barbers cut their own hair, while some not. So the perfect way to understand who is the good barber is asking someone in the town with a good hair cut.