Erbium - Periodic Table of Videos
The team at periodicvideos has created a TED-Ed Lesson for every element of the periodic table. In this one, the professor explains why erbium is perhaps the most important element for websites like YouTube.
Normally when you put energy into a material, its temperature rises. In 2006, it was reported by researchers at the University of the Basque Country that a glass of potassium lead chloride doped with erbium actually became cooler when a laser beam was focused on it.