Americans write the month first. A spy born on 5 June writes 5/6, which passes in either country. A spy born on 20 June writes 20/6, and there is no twentieth month.
During the Second World War, two German spies try to get into America posing as Americans coming home from abroad. Their papers are perfect. Both fill in the same form at the border. One is waved through and the other is arrested on the spot. Why?
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Look at what they wrote, not what they said.
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He wrote his date of birth the European way
Source: Agnes Rogers and Richard Sheehan, How Come, Again?, 1960