A travel mug usually has a spill-proof lid. This lid is designed to keep liquids from spilling out when cars go over bumps or as the result of the user's bouncy stride. The spill-proof design is also helpful for keeping heat inside the cup. Many have mechanisms that allow the user to take a quick sip of his or her beverage and reclose the lid with very little effort. This type of lid is often referred to as a spill-proof, drink-through lid. http://www.flowin-bottles.com/insulated-coffee-mug/thermo-travel-coffee-mug/