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Albert einstein was perhaps the best-known scientist of the twentieth centure. As well as his scientific work he also took an active interest in political and social problems.For example, in 1939 he wrote a letter to President Poosevelt encouragig the US to start building an atomic bomb.Later, when he found out about Hiroshima and Nagasaki, he changed his mind.`If I___(know), I___(become) a shoemaker,` he admitted.