Energy cannot be created or destroyed. 

      Energy can only be transformed from one form to another form.

      When energy is transformed / transferred, only part of it may be useful.

      Energy which is not transferred / transformed in a useful way is wasted.

      Waste energy is usually lost to the surroundings which become warmer.

        The light bulb is a device that transforms electrical energy into light and heat energies.

        The useful energy is light and the wasted energy is heat.

      Energy becomes increasingly more difficult to use for further energy transformations.