A boy propose marriage to a girl, and she uses the following way to make her decision:
She clenches six blades of grass in her right hand and shows only their heads as well as tails. Then she asks the boy to tie any two heads together and any two tails together. If these six blades of grass form a single ring after having been tied in the way described above, then the girl will decide to marry him.
Find out the probability that the boy is lucky.