A coffee coaster is a small, flat piece of material that is placed beneath a mug or glass of coffee to protect the surface of the table or countertop from heat or condensation. Coasters come in a variety of materials, such as cork, felt, plastic, wood, or even stone. Some coasters are designed to absorb the heat from the coffee mug, while others are designed to prevent the mug from slipping or scratching the surface of the table. Coffee coasters are also often decorated with designs or humorous phrases.