Andrew Ng and Daphne Koller, Stanford University computer science professors who started Coursera, pose for a photo at the Coursera office in Mountain View, Calif., on Aug. 2, 2012. Jeff Chiu/AP hide caption

itoggle caption Jeff Chiu/AP

Years ago, the boulder in the foreground broke loose and rolled on to this farm in northern Italy. Then this month, the boulder in the center of the photo tumbled down the mountain, tore through a barn and came to rest in a field. Fortunately, no one was hurt. Markus Hell/Tareom.com/AP hide caption

itoggle caption Markus Hell/Tareom.com/AP

A tasseled wobbegong shark (top) lies on the seafloor with the head of a brown-banded bamboo shark in its mouth on the fringing reef of Great Keppel Island on Australia's Great Barrier Reef in August 2011. Reuters/Landov hide caption

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Travelers at Pearson International Airport in Toronto earlier this month. At an unnamed airport, Canada's spy agency tested a program that allowed them to track those who took advantage of free Wi-Fi. Aaron Harris /Reuters/Landov hide caption

itoggle caption Aaron Harris /Reuters/Landov

Fireworks explode in a Shanghai street on Thursday, the eve of the Lunar New Year. Setting off firecrackers is meant to ward off evil spirits and bring good luck. But many Chinese say they won't buy firecrackers this year, owing to growing worries about air quality. Peter Parks/AFP/Getty Images hide caption

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Shoppers check out vodka in a street kiosk in Moscow in 2008. Alexander Nemenov/Getty Images hide caption

itoggle caption Alexander Nemenov/Getty Images