Democratic Rep. Henry Waxman of California stands in his Capitol Hill office beside a wall displaying his legislative accomplishments. Waxman, 74, said Thursday that he would retire after 40 years in the House of Representatives. J. Scott Applewhite/AP hide caption

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

Workers inspect an area near storage tanks where a chemical leaked into the Elk River at Freedom Industries storage facility in Charleston, W.Va., on Jan. 13. It is still unclear exactly how much of the chemicals leaked into the area's water supply. Steve Helber/AP hide caption

A mouse embryo grows from stem cells made by stressing blood cells with acid. The blood cells are tagged with a protein that creates green light. Courtesy of Haruko Obokata hide caption

An artist's concept of a narrow asteroid belt orbiting a star similar to our own sun. NASA/JPL-Caltech hide caption

Farmer Steve Henry of Arapahoe, Neb., looks at a patch of corn that failed due to drought last September. The new farm bill would expand crop insurance for farmers. Nati Harnik/AP hide caption

Graduate student Jennifer Klunk of McMaster University examines a tooth used to decode the genome of the ancient plague. Courtesy of McMaster University hide caption

Pete Seeger was an environmentalist, an activist and the most prominent folk musician of his generation. Neilson Barnard/Getty Images hide caption

Sen. Carl Levin (left) huddles with his brother and fellow Michigan Democrat, Rep. Sandy Levin, during testimony on the automotive industry bailout in 2008. Evan Vucci/AP hide caption

Activists hold signs during a rally for paid sick leave at New York's City Hall last year. Mary Altaffer/AP hide caption

With new construction in the background, President Obama spoke about manufacturing and jobs during a visit to Intel's Ocotillo facility, Jan. 25, 2012, in Chandler, Ariz. Ross D. Franklin/AP hide caption

A portion of Navajo artist Shonto Begay's mural depicting the Long Walk. The Bosque Redondo Memorial/Shonto Begay hide caption

Amanda Gerety, a staff nurse at Boston Medical Center, checks monitors that track patients' vital signs. Fewer beeps means crisis warnings are easier to hear, she says. Richard Knox/NPR hide caption

Alice Snyder, with her parents Mary and Ryan, during a checkup with Dr. John Herzenberg, who treated her clubfoot without surgery. Jenny Gold/Kaiser Health News hide caption