'Nerthikkadan' in Tamil means 'redeeming a promise made to a Diety'. A devotee would take a vow and promise to a Deity of his ('Ishta theivam') that he would do a certain act of penence if the Deity fulfills a wish made to the Deity. The act of penance can be a simple act like offering a 'poosai' at the temple of the Deity. It ranges from carrying a pot of milk on the head, carrying a decorated pot of milk on the head and dancing ('Karakam'), and carrying a 'Kaavadi' ('Kaavu, Thadhy' - carrying stick) It can take extreme forms like rolling on the ground following a 'Ther' called 'Angappradashanai'. One of the extreme forms is to get fish hooks piercing the skin of the back of the chest and being hung up on a pole following a procession. This act is called a 'Thookku kaavadi' or 'Paravaikkaavadi'. I took the above two pictures at a 'Ther' festival at the 'Muthumari Amman' temple in Kudagama road, Avissawella.
Click on link below to see video:-
http://www.youtube.com/watch?v=RoYWuqheOQY&feature=share&list=UU1dqepitfAJwrQWLD-gZ3NA