The Thais consider the head to be sacred and the feet profane. Touching someone's head, other than a child's, is taboo. A younger person or someone of lower social status will even lower their head in passing by a senior.
In sitting too, especially in the presence of monks or other exalted persons, attention must be paid to head level. Even more caution must be taken with the feet because of their contact with dirt. Similarly, the left hand is "polluted" in ritual meaning.
