Vacuum insulated mug provides the best heat retention. Vacuum means the air between the outer and inner layers of stainless steel has been removed to create a Vacuum insulation. It blocks heat from transferring through air (conduction). zo's Vacuum insulation also features a copper or aluminum layer tightly wrapped around the outside of the inner stainless steel layer. This copper or aluminum layer reflect heat (radiation), further minimizing the temperature change of your drink. Lastly, heat is blocked from escaping through the opening (convention) with a tightly closed lid. This is also ideal for cold retention. This mug is compact and has a lightweight design. The interior has a nonstick coding which resists odor and stains, making it a snap to clean. Boa free. Flip-open lid has a tight fit, is leak resistant and has a lid cover that maintains cleanliness where you drink from.