Guruji actually wrote down in Asa Di Vaar : Make compassion the cotton, contentment the thread, modesty the knot and truth the twist; This is the sacred thread of the Soul; if you have it then go ahead and put it on me; It does not break, it cannot be soiled by filth,it cannot be burnt or lost; Blessed are those mortal beings O Nanak who wear around their necks. Wikipedia