In Thailand, when a male reaches the age of 18 they are invited to participate in a ceremony of passage to adulthood, which is celebrated in the center of the village, surrounding the central fire and attended by the all the natives in residence, and accompanied by considerable consumption of food an drink amid singing, dancing and all manner of joyous festivities.
 
At the appointed time the native boys that are reaching maturity are formed into a large  circle around the fire and stand there motionless and stark naked facing inward towards the fire.

Then, accompanied by a haunting musical rendition of native music, the most beautiful girl in the village is honored to perform a sexy dance, naked, around the inner center circle.

Behind each boy is a naked native girl that he cannot see..  As soon as all the males are excited and have erections, the girls behind them reach between their legs, and pull their erect unit downward and back through and then on a signal all release them.

Their units spring back up and go loudly  "WHAP" against their bellies.  This is considered measurement of their strength and of youthful masculinity. And that's why the capital of Thailand is called Bangkok.