Smooth places within. The water flows and it seems it does not touch itself. That is the water has no friction within the water. Friction with the earth and sand and stones and then the friction lessens in time and the stone is smooth and the flow gentle. The flow of our life smooth. Bumpy friction from outside forces. Bumpy friction from within the self. Yet, smooth in reality.
The hollow bone, smooth and nothing on the inside to resist the transmission of spirit.