Indian schoolchildren write English alphabets on slates at a primary school outside Hyderabad in June. India is on track to overtake China as the most populous nation in just 16 years. Noah Seelam/AFP/Getty Images hide caption

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Volunteer John Kelley (left) leads an exercise class at a senior center in Oklahoma on Monday. Centers such as these offer seniors a cool place to go during heat waves. Sue Ogrocki/AP hide caption

itoggle caption Sue Ogrocki/AP

Bottles of antiretroviral drug Truvada, a medicine used in trials that showed a reduction in transmission of HIV between heterosexuals. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images