game.rb

class Game

  attr_reader :teams

  def initialize
    @teams = Array.new
  end

  def started?
  false
  end

  def innings
    Array.new
  end
  
  def add_team team
    @teams << team
  end

end

game_spec.rb

require File.expand_path('spec_helper', File.dirname(__FILE__))
require 'game'

describe Game do  
  context "before it is started", @game do
    it { should_not be_started }
    it { should have_no_teams }
    it { should have_no_innings }
    
    context "when a team is added" do
      before :each do
        subject.add_team mock('Team')
      end
      
      it { should have(1).teams }
    end
  end
end