Hestia the goddess of hearth,home,family,and the state. Hestia always sat next to a lit fireplace nowhere else unless she was beside a fireplace. This became a problem see Poseidone always wanted to marry Hestia. So this is when Poseidone decide to take Hestia. The only way Poseidone would let her go back to the lit fireplace was if she married him. Which was a problem for Hestia because she promised Zeus she would never enter a union with a man. So she had a decision to break a promise or never see the lit fireplace again which she also needs to survive. SO Hestia decide to marry Poseidone so she can see the lit fireplace again. Not only was Poseidone happy but Zeus was angry at her she was one of the three that was not going to marry. So
