The Senninkotsu's basic makeup is a long head,sturdy bones and not many thing are narrow.
Although most with the Senninkotsu become Sennin, this does not happen for everyone. Only those scouted by the Kongrong Mountains or Kingo Islands become Sennin. The few that aren't become Tennen Doshi.
How to get it Edit
The Senninkotsu is usually present in a person from birth. However, a person with the Senninkotsu will not always pass it on. Examples of this are King Chu who came from a family known for producing Sennin but did not have it and Hiko Ko who was a Tennen Doshi but only passed the Senninkotsu on to Tenka Ko and Tensho Ko. The other ways of obtaining it are to have it placed in you by a Sennin (rarely done due to the risk) or in Bunchu's own words "Train until your body rots".