A stainless mug is a vacuum insulated mug that is designed to drink directly from the mug. Although similar to travel mugs, it offers better heat retention because of its tight fitting lid, and can be stored in a bag or purse without spilling. How does it keep beverages hot? The air between two thin layers of stainless steel that make up the body of the mug is removed to create a vacuum. Because there is no air, the temperature on the other side of the wall does not transfer over, keeping beverages hot or cold for hours.