Learn how glass—a material usually considered fragile, and dangerous if shattered—can be made into a strong, safe wall in this video from NOVA Online. Several treatment methods were used for the glass panes that make up the new One World Trade Center building. Heat tempering makes the glass tough enough to handle severe weather conditions. A polyurethane coating serves two purposes:first, to keep the glass from shattering upon impact with another object; and second, to keep the glass from getting too hot from the sun's rays. The second purpose also enables the building to be super green and LEED certified.