At 3D theaters, you’re handed lightweight passive glasses that work in tandem with a polarizing filter positioned over the projector’s lens. When viewing at home with a 3D TV, you use bulky, battery-powered glasses with active shutter liquid-crystal lenses. Passive glasses in theaters are cheap and easily replaced. But at an average cost of $100 per pair, glasses used at home represent a sizable investment. Better to put them in a safe place — and keep ’em away from kids!