Sobibór extermination camp
When the Germans invaded in May 1940, Klein began working in a Jewish hospital and continued with his doctoral studies in 1941. In 1942, though, he had to hide; his brother was captured and taken to the Sobibór extermination camp, where he died. After the war, Klein returned to his doctoral studies, but he also worked in circuses in France, Belgium, and the Netherlands, performing fast calculations as an act, often under the stage name 'Pascal'. He lived a fairly nomadic lifestyle and performed in such shows until 1952.