spec.rb

class Hero
  attr_reader :hp

  def initialize
    @hp = 100
  end

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

  def heal!(amount)
    @hp += amount
  end
end

describe Hero do
  subject { Hero.new }

  it "should lower hp when they're hit" do
    subject.hit!(10)
    subject.hp.should == 90
  end

  it "should raise hp when they heal" do
    subject.heal!(10)
    subject.hp.should == 110
  end
end