The head is the most important part of the entire body. It should never be touched. Even in jest, you should resist the affectionate casual ruffling of a youngster's hair to show compliments, the way it is done outside Thailand. This is why a layman will sit so that his head is lower than that of a monk's and a younger person will do likewise to an elder person or someone of a higher rank, status or position. The lower the head, the greater the respect given.