Imagine standing on a glass platform with Chicago 1300 feet directly below. Suddenly, the glass holding you begins to crack. This actually happened to Alejandro Garibay at the Willis Tower (formerly the Sears Tower) just last week. Luckily, Garibay wasn't hurt, but the occurrence begs the question: how safe is glass - the most common material used in skyscrapers nowadays - really? Karrie Jacobs At Fast Company - Design, asked materials experts to find out "The Truth Behind Building With Glass."