The highly rated variety of medical marijuana known as "Blue Dream" was displayed among other strains at a cannabis farmers market in Los Angeles in 2014. Frederic J. Brown/AFP/Getty Images hide caption

For the first time, scientists have synthesized a three-stranded molecular braid that twisted into a knot with eight crossings, as in this rendering. Stuart Jantzen/Biocinematics.com/Science hide caption

The killer whale J2, better known as "Granny," pokes her head out of the water in the Salish Sea near the San Juan Islands of Washington in July 2016. Granny, who was thought to be about 105 years old at the time, was presumed to have died later that year. Mark Malleson/Center for Whale Research/AP hide caption

The hyolith Haplophrentis extends the tentacles of its feeding organ (lophophore) from between its shells. The paired spines, or "helens," are propping the animal up off the ocean floor. Danielle Dufault/(C) Royal Ontario Museum hide caption

Despite efforts to reduce pedestrian deaths, many cities have become more dangerous for walkers in recent years. Alessandro Scagliusi / EyeEm/Getty Images/EyeEm hide caption

This 52-million-year-old fossilized tomatillo was found in Patagonia, Argentina, shedding light on the origin of nightshade plants. In this specimen, the slender stalk is preserved, and the former papery and lobed husk is broken at top to reveal the large, fleshy berry underneath — now turned to coal. Peter Wilf, Penn State University hide caption

Dungeness crabs for sale at Fisherman's Wharf in San Francisco. California's Dungeness crab season was shut down in 2015, when record high ocean temperatures and lingering toxic algae blooms raised the domoic acid in shellfish to unsafe levels. A new study links dangerously high levels of the neurotoxin to warmer ocean temperatures, suggesting such closures could become more common in the future. Eric Risberg/AP hide caption

Scientists predict a star collision in the constellation Cygnus. NASA/IPAC/MSX hide caption

Brandie Jefferson Courtesy of Lopestagram/Instagram hide caption

Scientists used the National Radio Astronomy Observatory's Very Large Array near Socorro, N.M., to detect fast radio bursts. Patrick Semansky/AP hide caption

A child takes a facial recognition test in which he is asked to match the face on the top to one of the faces on the bottom. Jesse Gomez and Kalanit Grill-Spector at the Vision and Perception Neuroscience Lab/Science hide caption

Researchers from the Max Planck Institute excavate the East Gallery of Denisova Cave in Siberia in August 2010. With ancient bone fragments so hard to come by, being able to successfully filter dirt for the DNA of extinct human ancestors can open new doors, research-wise. Bence Viola/Max Planck Institute for Evolutionary Anthropology hide caption

Having volunteers who are learning German answer questions about grammar and semantics of the language while inside an MRI machine might show particular patterns in brain changes, researchers say. They hope their study could offer clues to how the brain best learns a second language. selimaksan/Getty Images hide caption

A natural gas drilling rig's lights shimmer in the evening light near Silt, Colo. Methane is the main component of natural gas, and studies show some methane escapes from leaky oil and gas operations. David Gilkey/NPR hide caption

Emil Girardi, 83, and Shipra Narruhn, 67, chat in Girardi's San Francisco apartment. They were paired through a nonprofit called Little Brothers, Friends of the Elderly, which aims to relieve isolation and loneliness. Anna Gorman/KHN hide caption