In olden days, when Thailand was Siam there was no order of succession as the king had many wives and many sons.
When the king died all the princes had to form a line in the palace courtyard and remove all their clothes
The Lord high chancellor would move along the row of naked princes and place a small dab of honey on each navel. It is of course hot there and soon the honey attracted flies
A beautiful dancing maid would then perform a seductive dance and move along the line and slap each man's winkie in turn so that it rebounded. The first prince whose rebound trapped a fly was declared the new king.