How do you keep heat in a thermos?

Heat is retained in a Styrofoam cup bu stacking at least 2 and place a lid. Because Styrofoam is well insulated, it retains the heat, and by added a minimum of 2 cups, it increases the amount of insulation. It is also important to note, that some heat will still be lost, and you will be able to tell if you can feel the heat through the outer cup.