President Obama speaks during an April 7 visit to Bladensburg High School in Bladensburg, Md. It was his fourth visit to Prince George's County in as many months. Aude Guerrucci-Pool/Getty Images hide caption

An artist's rendering of Kepler-186f, the first validated Earth-size planet to orbit in the habitable zone of a distant star. T. Pyle/NASA/SETI Institute/JPL-Caltech hide caption

The Secret Sisters' new album, backed by Jack White and produced by T-Bone Burnett, is called Put Your Needle Down. Courtesy of Republic Records hide caption

This mouse egg (top) is being injected with genetic material from an adult cell to ultimately create an embryo — and, eventually, embryonic stem cells. The process has been difficult to do with human cells. James King-Holmes/Science Source hide caption

Admirers ask Gabriel Garcia Marquez --€” seated alongside his wife, Mercedes Barcha €-- to sign books in Santa Marta, Colombia, in 2007. Alejandra Vega/AFP/Getty Images hide caption

While many women continue to work with little change in their duties while pregnant, others find that pregnancy can be a career liability. Yuri Arcurs/iStockphoto hide caption

The Serpent Mound in southern Ohio is 3 feet high and more than 1,300 feet long. Courtesy of the Ohio Historical Society. hide caption