Telescopes allow astronomers to view distant objects in space. Some of these instruments focus light using mirrors coated with a thin layer of the mystery element. To make a telescope mirror, the mystery element is heated inside a vacuum, or airless space. The metal evaporates, turning into gas, and its molecules settle on the mirror. When cooled, the coating solidifies, giving the mirror a shiny, reflective surface, explains Andrew R. Barron, a chemistry professor at Rice University in Texas. Know the answer? Score 80 points. Otherwise, go to clue 3.