Thermal energy is transferred to the surroundings as it is conducted through the sides of an object. A pan is made from metal, which is a good conductor of heat, but unfortunately, this means that thermal energy is transferred to the surroundings very easily. Kettles are normally made from plastic. Plastic has a lower thermal conductivity than metal and so less energy is transferred to the surroundings. This makes a kettle more efficient than a pan for boiling water.