What do you get when you power a layer of liquid crystal material sandwiched between two sheets of glass? A Liquid Crystal Light Valve (a.k.a a LCD Controllable Black-out Panel)! They're often used for electric welding helmets because they can protect the welder's eyes from the bright sparks. Or – if you aren't a welder – they can be used to provide a "reveal" effect for your project, as you can control the opaqueness of the LCD.