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A penlight is a small flashlight in the shape of a pen. A penlight usually uses a small light known as an "LED" (Light Emitting Diode) but it can also use a small lightbulb made just for small flashlights.
The LED lights up when power is run through it. Making the penlight light up is as simple as pressing a button, or screwing the end on tighter so it makes contact with the batteries.en:Penlight