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This is an extremely simple, but cool idea for a coaster. It works by placing small watch batteries inside an acrylic coaster. Then connect one end of the LEDs to the battery while bending the other LED into an L-shape. Once a full, frosty beverage is placed on the coaster, the other LED contact will touch the battery and light up. Follow the linkage for the detailed instructions, including pictures.

CoasterGlow [MetkuMods]