Answer: 1. An isolated system is one in which neither energy nor mass is allowed to cross the boundary of the system.
2. Let's have an imaginary thermos flask insulated with a material of infinite thermal resistance.
3. This Thermos flask when closed with a knob can not transfer mass and energy.
4. Obviously, this is an imaginary system because in practical engineering applications there does not exist any material with an infinite thermal resistance (or zero thermal conductivity)