Professional dog walker Jon Lovette of Jonny Walkers walks with dogs at Crissy Field on October 19, 2011 in San Francisco, California.

Teachers carry children away from Briarwood Elementary school after the tornado destroyed the school in south Oklahoma City. (Photo by Paul Hellstern/The Oklahoman)

A pile of destroyed cars of teachers sits outside Briarwood elementary school in Oklahoma City, Oklahoma May 22, 2013. Rescue workers with sniffer dogs picked through the ruins on Wednesday to ensure no survivors remained buried after a deadly tornado left thousands homeless and trying to salvage what was left of their belongings. Curvature of horizon in the photo is due to an ultra-wide angle lens. (Photo by Rick Wilking/Reuters)

A traffic policeman hides a rose behind his back before giving it to a female driver ahead of International Women's Day in Ryazan, Russia on March 7, 2017. (Photo by Alexander Ryumin/TASS via Getty Images)

The second and final qualification motor (QM-2) test for the Space Launch System's booster, June 28, 2016, at Orbital ATK Propulsion Systems test facilities in Promontory, Utah. During the Space Launch System flight the boosters will provide more than 75 percent of the thrust needed to escape the gravitational pull of the Earth, the first step on NASA's Journey to Mars. (Photo by Bill Ingalls/NASA)

In this handout image provided by Ogilvy, a burger made from cultured beef, which has been developed by Professor Mark Post of Maastricht University in the Netherlands (pictured) is shown to the media during a press conference on August 5, 2013 in London, England. The in-vitro burger, cultured from cattle stem cells, the first example of what its creator says could provide an answer to global food shortages and help combat climate change, was fried in a pan and tasted by two volunteers. The burger is the result of years of research by Dutch scientist Mark Post, a vascular biologist at the University of Maastricht, who is working to show how meat grown in petri dishes might one day be a true alternative to meat from livestock.The meat in the burger has been made by knitting together around 20,000 strands of protein that has been cultured from cattle stem cells in Post's lab. (Photo by David Parry)

Smithsonian National Zoo biologist Leigh Pitsko releases a male lion cub for its swim test in the zoo habitat moat, in Washington May 6, 2014. Four, unnamed ten-week old lion cubs were tested today for their ability to swim and remove themselves from their zoo habitat moat. (Photo by Gary Cameron/Reuters)