def down(state):
    """move blank space down on the board and return new state."""
    new_state = state[:]
    index = new_state.index(0)
    if index not in [6, 7, 8]:
        temp = new_state[index + 3]
        new_state[index + 3] = new_state[index]
        new_state[index] = temp
        return new_state
    else:
        return None