Light is on when the cup is filled with liquid (water, drink, alcohol, etc.), and off when no liquid's filled in; Flashing thythm is changeable automatic; Cleaning with water; Suitable for bar / night club / restaurant / party / house decoration; Also good gift for your best friends; Water inductive and rainbow color light; Very shiny and cool; Powered by built-in 2 x CR2025 batteries, replaceable.