UV is short for ultraviolet. It is a kind of light your eyes cannot see, sitting just past the violet end of the rainbow — that is exactly what "ultra-violet" means: beyond violet. It comes from the sun, and it is the part of sunlight that tans your skin, burns it, ages it, and over years can cause skin cancer. It is also what makes glow-in-the-dark posters glow and what banknote checkers shine on money to see the hidden marks. In everyday English "UV" is used three ways. As a noun: "too much UV is bad for you." As an adjective before another noun: "UV light," "UV rays," "UV protection," "a UV index of 11." And in fixed labels you will see on products here in Taiwan: "UV cut," "anti-UV," "UPF 50+." People say the two letters separately, "you-vee," never "uhv." One thing worth knowing: UV goes through clouds and glass differently from ordinary light, which is why you can burn on a grey day and why drivers get more sun damage on one arm. That is why the weather report gives a separate UV index from the temperature — a cool day can still be a high-UV day.