The Mathare Valley, shown here in an aerial map, is one of the largest and oldest slums in Nairobi, Kenya. Residents are using hand-held GPS devices to map the area, which comprises 13 villages and is home to nearly 200,000 people. Courtesy of Muungano Support Trust and Jason Corburn, UC Berkeley hide caption

Wood fibers are coated with carbon nanotubes and then packed into small disks of metal. The sodium ions moving around in the wood fibers create an electric current. Heather Rousseau/NPR hide caption

Wheat grows in a test field at Oregon State University in Corvallis. Some scientists believe that there's a chance that genetically modified wheat found in one farmer's field in May is still in the seed supply. Natalie Behring/Bloomberg via Getty Images hide caption

Erik and Mary-Ann Gjermundsen of Norway are making the nearly 3,400-mile trip across the U.S. in their small Messerschmitt. Jenna Dooley/NPR hide caption

Red blood cells are normally shaped like doughnuts, but sickle cells (purple) are flattened and clump together. NIH hide caption

Mattel executives say they did not anticipate the runaway success of the goth-influenced Monster High brand when it debuted in 2010. Mattel hide caption

Nearly 100 years after Edouard Manet painted his scandalous 1863 Olympia, Tom Wesselmann created The Great American Nude #26. Estate of Tom Wesselmann/Licensed by VAGA, New York, NY hide caption

Davis says that in his opinion, Roger Maris (above) is the all-time home run record holder. Maris hit 61 homers in 1961. AP hide caption