Liquid crystal display
Each pixel (block) of an LCD is made of a thin layer of molecules between two electrodes and two polarizing filters. The electrodes provide electric power to the liquid crystal layer, and don't block the light.  Light travels with 'polarity' or direction, and a polarizing filter only lets light with one kind of polarity to go through it, like trying to slide a ruler through a narrow opening.  Only when the ruler is lined up right, will it fit.  These two filters are perpendicular to one another, so the narrow openings are in different directions. This means that without the liquid crystals between them, they would block all light from passing through – whatever light gets through the first filter will not fit through the second filter.