Numbers.
There are mystic qualities attached to numbers. Thus, when Hindus have removed the ashes from a burning ground they write the figures 49 on the spot where the corpse was cremated. The Pandits explain this by saying that when written in Hindi the figures resemble the conch-shell and wheel of Vishnu, or that it is an invocation to the forty-nine winds of heaven to come and purify the ground. It is more probably based on the idea that the number seven, as is the case all over the world, has some mystic application. So in the folk-tales the number three has a special application to the tests of the hero who endures the assaults of demons or witches for three successive nights. The idea of luck in odd numbers is universal, and the seventh son of a seventh son is gifted with powers of healing.