Liskov Sustitution Principle: Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T.

lsp.rb

# Violates LSP
class Company
  def hire_people
    expand_the_team
  end

  def open_new_location
    create_a_new_office
  end

  def make_money
    "$$$$$"
  end
end

class Corporation < Company
  def stock_sale
    call_more_stock_holders
  end
end

# Enforces LSP
class Company
  def hire_people
    expand_the_team
  end

  def open_new_location
    create_a_new_office
  end

  def make_money
    "$$$$$"
  end

  def stock_sale
    raise NotImplementedError
  end
end

class Corporation < Company
  def stock_sale
    call_more_stock_holders
  end
end