Frank resigning from his post at the University of Minnesota. After being branded a Communist, Oppenheimer could no longer find work in physics in the US, and he was also denied a passport, preventing him from working abroad. Frank and Jackie eventually sold one of the Van Gogh paintings he had inherited from his father, and with the money bought of ranch land near Pagosa Springs, Colorado, and spent nearly a decade as cattle ranchers. Return to teaching In 1957, the Red Scare had lessened to the point that Oppenheimer was allowed to teach science at a local high school. Under Oppenheimer's tutelage, several students from Pagosa Springs High School took first prize at the Colorado State Science Fair. Within two years, supported by endorsements from physicists Hans Bethe, George Gamow, and Victor Weisskopf, Oppenheimer was offered a position at the University of Colorado teaching physics. While returning to particle physics research, Oppenheimer also took an increasing interest in developing improvements in science education. He was eventually awarded a grant from the National Science Foundation to develop new pedagogical methods, which resulted in a "Library of Experiments"—nearly one hundred models of classical laboratory experiments which could be used in aiding the teaching of physics to elementary and high school children. These models would later become the core of the first exhibits at the Exploratorium. Oppenheimer also worked with the Physical Science Study Committee (PSSC), helping to develop a new high school physics curriculum in the immediate post-Sputnik years. In his work, Oppenheimer followed the well-known old Latin principle Docendo discimus—"the best way to learn is to teach". Exploratorium In 1965, Oppenheimer was awarded a Guggenheim Fellowship to study the history of physics and to conduct bubble chamber research at University College, London, where he was exposed to European science museums for the first time. Inspired, Frank devoted the next years of his life to creating a similar resource in the United States. Upon his return from Europe, he was offered a job planning a new branch of the Smithsonian Institution in Washington, DC, but he instead chose to work on setting up a completely independent new type of museum in San Francisco. Four years later, in 1969, the Exploratorium opened its doors for the first time—an interactive museum of art, science, and human perception based on the philosophy that science should be fun and accessible for people of all ages,