In this July 25, 2015 photo, youth eat a cooked pig's head as they wait for the start of carnival in Santiago, Cuba. (Photo by Ramon Espinosa/AP Photo)

People try to board a goverment bus during the second day of a suspension of public transport services in Apopa, El Salvador July 28, 2015. Violent gangs in El Salvador ordered bus drivers to strike on Monday in a conflict that has killed six people and left thousands of commuters stranded on the streets of the Central American capital, police and bus company officials said. (Photo by Jose Cabezas/Reuters)

Sorana Cirstea of Romania hits the ball against Jie Zheng if China during the Sony Ericsson Open at Crandon Park Tennis Center on March 22, 2011 in Key Biscayne, Florida.

Bus 392 stands on the highway running from Beersheva to Eilat, with windows broken following a gun attack on August 18, 2011 near the Israel - Egypt border, Israel. A series of coordinated gun and roadside bomb attacks against miltary and civilian targets near the Israeli - Egyptian border, including an ambush on the bus travellling 20km north of Eilat, are reported to have left 5 dead and many injured. (Photo by Yehuada Ben Itah/Getty Images)

These “snapcats” have found their bunny doppelgängers in a series of sweet photo shoots. Animal photographer Mark Taylor, from Surrey, England, and his assistants have scoured the U.K. to bring together carbon-copy rabbits and cats. Here: Chocolate Birman-cross kitten with chocolate Lop rabbit. (Photo by Mark Taylor/Warren Photographic/Caters News Agency)

Everyone wants to get something for nothing - whether it's a lucky upgrade that means you can turn left when you get on a plane instead or heading off into the economy seats, or even getting a few extra features thrown in for free when you buy a new car.

As some of the sharpest businesses around, no-one understands this quite as well as casinos and that's why they all have a system that they call comps. It's short for "complimentary offers" and these are special treats specially designed to reward you for your loyalty to the casino in question. They're basically bonuses.

Electric cars sit charging in a parking garage at the University of California, Irvine January 26, 2015. “The Irvine Smart Grid Demonstration”, a $79 million project funded half by federal stimulus money and half by Edison and partners like UC Irvine, was launched in 2010. The $12 billion utility's research team Southern California Edison is testing everything from charging electronic vehicles via cell phone to devices that smooth out the power created by rooftop solar panels. Those are some of the roughly 60 projects in the works at Edison's Advanced Technology division. It has a small $19 million annual budget, but its influence far exceeds that. (Photo by Lucy Nicholson/Reuters)

The supermoon rises behind Glastonbury Tor on September 27, 2015 in Glastonbury, England. Tonight's supermoon, so called because it is the closet full moon to the Earth this year, is particularly rare as it coincides with a lunar eclipse, a combination that has not happened since 1982 and won't happen again until 2033. (Photo by Matt Cardy/Getty Images)