The story of how a father and son team - one a physicist, one a geologist - helped solve a big scientific mystery. What brought the reign of dinosaurs to an end? NPR Science Correspondent Richard Harris tells us how they turned to an element, iridium, for answers. Plus, how iridium could help prevent another potential future global catastrophe. It's our celebration of 150 years of the periodic table of elements. Follow Maddie on Twitter @maddie_sofia. Email the team at shortwave@npr.org.