The grooved pattern holds liquid to prevent leaking and promotes air flow to prevent coaster from sticking to glassware. A great concave design lets the OXO Good Grips Coasters contain liquids from condensation build-up. Made of sturdy silicone, the set of 6 coasters is completely dishwasher safe. Use either side, since the concave design is same on top and bottom. The coasters are available in a choice of colors for easy accessorizing.