Parent
father or mother
Does a person need to be a parent to become a grandparent?
Parents care for their children. When the children grow up and have kids of their own, the parents become grandparents to those kids. A person who is not a parent has no kids, therefore nobody to produce grandchildren for them.
What must a person have in order to be known as a grandparent? What would the parents of #1 be to the person? Must one be a parent to have #2?