In the 20th century, new types of <answer>glass</answer> such as laminated <answer>glass</answer>, reinforced <answer>glass</answer> and <answer>glass</answer> bricks have increased the use of <answer>glass</answer> as a building material and resulted in new applications of <answer>glass</answer>. Multi-storey buildings are frequently constructed with curtain walls made almost entirely of <answer>glass</answer>. Similarly, laminated <answer>glass</answer> has been widely applied to vehicles for windscreens. While <answer>glass</answer> containers have always been used for storage and are valued for their hygienic properties, <answer>glass</answer> has been utilized increasingly in industry. Optical <answer>glass</answer> for spectacles has been used since the late Middle Ages. The production of lenses has become increasingly proficient, aiding astronomers as well as having other application in medicine and science. Glass is also employed as the aperture cover in many solar energy systems.
The walls of skyscrapers are often made of what material?
glass