gistfile1.rb

class Hero                                                                                                                                             
  attr_reader :hp 

  def initialize
    @hp = 100 
  end 

  def hit!(force)
    @hp -= force
  end 
end

describe Hero do
  subject { Hero.new }

  it "should lower hp when they're hit" do
    lambda {
      subject.hit!(10)
    }.should change(subject, :hp).by(-10)
  end 
end