 On Saturday, 15th of October, ESO, the European Southern Observatory, opened the doors of its headquarters in Germany to the public. Throughout the day, thousands of visitors had the chance to help build a full-size mock-up mirror of the largest planned telescope in the world, the European Extremely Large Telescope, or EELT, and to experience many other aspects of ESO's work. This is the ESOcast, cutting-edge science and life behind the scenes of ESO, the European Southern Observatory, exploring the ultimate frontier with our host Dr. J, aka Dr. Joe Liske. ESO's 2011 Open House Day attracted around 3,100 visitors to its headquarters in Garching near Munich, Germany. The visitors were curious to find out more about ESO's world-class facilities and frontline scientific results. Participants had a wide variety of activities to choose from, with the highlight of the event being the construction of a live-size cardboard mock-up of the EELT's giant 39.3-meter mirror. This gave participants a unique chance to experience the true scale of the world's future biggest eye on the sky. Because the EELT's main mirror is so large, it consists of 798 individual hexagonal mirrors, each about 1.4 metres across. Using pegs and an outline of the mirror on the ground, enthusiastic visitors of all ages place the cardboard hexagons on the ground to slowly form the giant mirror. When finished, the full-size mock-up measured 39.3 metres across and occupied an area of nearly 1,000 square metres next to the ESO headquarters. Additionally, there were talks on hot topics in astronomy and video links to the very large telescope, giving people the opportunity to chat live with ESO astronomers working on Cerro Paranal in the Chilean Atacama Desert. Children and their parents enjoyed having their photos taken in infrared light and observing the sun through telescopes equipped with suitable filters. Three exhibitions were dedicated to ESO's very large telescope, the Atacama Large Millimeter Sub-Millimeter Array and the future EELT. Once again, the Open House Day at ESO was a great opportunity for people to discover the latest exciting developments from the world's most productive astronomical observatory. This is Dr. Jay signing off for the ESOcast. Join me again next time for another cosmic adventure.