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Adrian Chen answers: " .... With a computer. In a typical reconstruction process, technicians feed all the available shreds into a scanner. An automated software program then assigns a unique ID to each piece and analyzes a number of characteristics, including size, color, indentation, and font. Using a matching algorithm, the software then identifies potential neighboring shreds, displaying them onscreen for an operator to confirm.Not all shredded documents can be put back together. The possibility and ease of reconstruction depends on the size of the shreds—the smaller and more numerous the pieces, the harder it is to reconstruct a document. Strip shredders cut paper into long strips 1/8- to 5/16-inch wide and are the most popular option because of their speed and low cost—but they're also the easiest to reassemble since they produce a relatively small number of large fragments. Cross-cut shredders, which slice paper into many tiny, confettilike pieces, are significantly more secure (and expensive), while shredders that pulverize paper into dust cost thousands of dollars but are essentially reconstruction-proof. (To secure top-secret information, the Department of Defense requires that the majority of shredded particles "not exceed 5 square millimeters.") When using a strip-shredder, the slicing direction also has implications for reconstruction. Horizontal cuts may leave entire lines of text intact. Vertical shredding, which ensures that sentences are broken up, is more secure.Before advances in scanning and computer technology, documents had to be reconstructed by hand. Assuming all the pieces are in one place, reassembling a shredded document is a bit like solving a jigsaw puzzle; the reconstructionist must painstakingly sift through the shreds, looking for matches. During the 1979 Iranian Revolution, students and militants who took over the U.S. Embassy in Tehran turned to local carpet weavers to reassemble classified CIA documents they found that had been shredded. These pages were later published in a set of about 60 volumes called Documents From the U.S. Espionage Den. And in 2002, former FBI agent William Daly took about an hour to reassemble a shredded page from the dictionary on Good Morning America."Link: http://www.slate.com/id/2223844