Classes are Just Objects

Class Interception:
- Create a "Proxy Class" object
- Mirror the class name with a class constant pointing to the proxy
- Wrap it all up in a pretty interface called use_class

  def test_warmer
    Warmer.use_class(:Heater, MockHeater) do
      # Here, anytime a method in Warmer references Heater
      # it will get a MockHeater class instead.
    end
    # Here Warmer is back to normal.
  end

(source)