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.

These Northern Lights “rainbows” have turned the sky greener than the Earth below. Vitaly Istomin, 26, spent several nights in freezing conditions under the stars in northern Russia’s Khibiny Mountains to capture the aurora’s “rainbows”. (Photo by Vitaly Istomin/Caters News Agency)

Aerial photo taken on March 4, 2017 shows cole flower fields in Luoping County, southwest China's Yunnan Province. (Photo by Xinhua/Barcroft Images)

A reveller plays with tomato pulp during the annual Tomatina festival in Bunol, near Valencia, Spain on August 29, 2018. As every year on the last Wednesday of August, thousands of people visit the small village of Bunol to attend the Tomatina, a battle in which tons of ripe tomatoes are used as weapons. This year, a total of 145 tons of ripe tomatoes will be thrown between more than 22,000 participants. (Photo by Heino Kalis/Reuters)

Hao Han (8), Shaolin kung fu student and child actor. Shaolin Monastery, Henan. (Photo by Mathias Braschler and Monika Fischer)

Fisherwomen laying barrels with salt in preparation for herring curing, 1937. (Photo by Topical Press Agency)

Pod of dolphins surf effortlessly through the waves in the wild traveling at speeds up to 20 mph. (Photo by Greg Huglin)