Opossums on their mothers back at a game reserve in Central Minnesota, USA, on September 2, 2013. (Photo by Caters News Agency)

Greyhounds with stuffed and sprung model monkeys strapped to their backs before a hurdle race at Wellinborough. (Photo by E. Bacon). 21st April 1928

A mouse rides on the back of a frog in floodwaters in the northern Indian city Lucknow June 30, 2006. (Photo by Pawan Kumar/Reuters)

A humanoid robot named “Robi” sits during a promotional event for the launch of a weekly Robi Magazine in Tokyo, Japan, 20 January 2015. Created by roboticist Tomotaka Takahashi, the 34-centimeter tall humanoid robot can recognize and respond to more than 200 words and phrases, walk and dance. Parts of the robot will be put on sale in a weekly magazine to be fully assembled with 70 issues. (Photo by Franck Robichon/EPA)

In a 200-acre-plus dump 5 kilometers north of Haiti’s capital, Port-au-Prince, hundreds of men, women and children scavenge day and night through the burning wasteland. They earn $12 to $15 a day – on a good day – for recycling plastics as well as clothing, household items and aluminum (for smelting). Some 5,000 tons of waste is created each day in the Port-au-Prince area. (Photo and caption by Giles Clarke/Getty Images Reportage)

The claws are out for North Korean dictator Kim Jong-un and Russia's Vladimir Putin – as cats now able to use a model of him as a scratching post. And moggies can also maul at Russian president Vladimir Putin, whose face also features on the new cat toys which are 1.5ft tall and cost £4,500. They are made from hessian rope, and 3D-printed faces are then attached to the posts, before they are handpainted. The toys took a team of artists 200 hours to finish. (Photo by The Pussycat Riot)

A military guard observes from a watch tower at the Combinado del Este prison during a media tour in Havana, Cuba, Tuesday, April 9, 2013. Cuban authorities led foreign journalists through the maximum security prison, the largest in the Caribbean country that houses 3,000 prisoners. Cuba says they have 200 prisons across the country, including five that are maximum security. (Photo by Franklin Reyes/AP Photo)

A view motorists have grown accustomed seeing, a military truck made by Oshkosh Corp. is taken for a test drive Thursday, April 11, 2013 in Oshkosh, Wis. Faced with deep cuts in U.S. military spending, and the end of the wars in Iraq and Afghanistan, Oshkosh Corp. is laying off 900 employees in its defense division based in Oshkosh. Approximately 700 hourly workers at the state's largest manufacturer will lose their jobs in mid-June, followed by approximately 200 salaried employees through July. (Photo by Mark Hoffman via The Journal Sentinel)