The primary functional purpose of a drink coaster is to protect the surface of a table or furniture from the damaging effects of "sweating" or condensation produced by cold beverages. When a cold glass sits in a warm room, moisture from the air condenses on its exterior and pools at the base; without a coaster, this water can cause permanent white rings on finished wood, warp veneers, or leave stains on porous stone. Coasters made from absorbent materials like pulpboard, cork, or unsealed terracotta actually soak up this moisture, while those made of silicone or slate act as a barrier to keep the liquid off the table. Beyond moisture protection, they also serve to insulate the surface from the high heat of mugs containing coffee or tea, which can scorch or crack delicate finishes. In a social setting, placing a coaster on top of your glass is a common "pro" signal to servers that you are not finished with your drink, and in pubs, they often serve as a canvas for branding and advertisements.