What would you collect if money were no object?
Football teams? Who needs the hassle?
Caribbean islands? So 1980s.
How about a $1.8m soft toy collection? Now we're talking. And that's exactly what hedge fund manager Paul Greenwood decided on, in addition to the odd car and horse of course.
Think the rarest Steiff bears in the world - the pinnacle of stuffed toy collecting - including one of only 10 Hot-Water Bottle Bears ever made and a never released Bonzo the Dog toy.
With money no object, you see, you are free to feed your passions.
Yet unfortunately for Greenwood, it wasn't his money he was feeding his passion with.
Because earlier this year Greenwood was found guilty of defrauding investors of $554m - and that $1.8m sale of his toy collection was to pay his legal fees.
That's not to suggest that the 1,592 US dollar billionaires in the world have achieved their wealth in anything other than the most upstanding of ways.
But it got us thinking. What do the world's billionaires collect? What do you buy when you could have anything you wanted?
We're about to find out:
5 billionaires who collect
Bill Koch
Worth: $4bn
Collects: Wild West and maritime memorabilia
The four sons of Fred Koch - the founder of Koch Industries - are all wealthy men. Charles and David kept the family business going and are now worth more than $30bn each. They are well known for their championing of Republican causes. William and Frederick, however, are the poor relations - and devote much of their time to building their collections.
Bill, 74, has just $4bn to his name. His big passion is maritime memorabilia, unsurprising considering he won the America's Cup in 1992 with his self-financed team - even sailing on the boat himself. His collection includes the Italian boat he defeated, as well as antique nautical instruments, model ships and seascapes.
Yet he's far from a one trick pony. Bill also bought the only authenticated photograph of US outlaw Billy the Kid for $2.1m in 2011 to complement the Jesse James gun and General Custer flag that form his million piece collection of Wild West memorabilia. They no doubt all feature in the private Wild West-style town Bill has built in Colorado. Bear Ranch features five saloons, a brothel (for guests to stay), and a church - for the morning.
 Bill Koch paid $2.1m for the only authenticated photo of Billy the Kid |
Bill also has a number of wine bottles said to have belonged to Thomas Jefferson. Their authenticity is suspect. They were sold to Bill by one Rudy Kurniawan, imprisoned earlier this year for selling fake fine wine.
Frederick took his cut of $330m of the family business in the 1980s so is a long way from being a billionaire. But that hasn't stopped the most artsy of the four brothers from financing the $2.8m upgrade to The Swan theatre in Stratford-upon-Avon, or forming one of the finest manuscript collections around. His collection includes handwritten scores from Schubert and Mozart, working drafts by Oscar Wilde and poems by Victor Hugo, reports Vanity Fair.
Bruce Halle
Worth: $4.9bn
Collects: Tyre posters - has the world's largest collection
Bruce Halle has spent his life with tyres. In 1960 he opened his first store, in Michigan, with a stock of just six used tyres. Today Discount Tire employs more than 15,000 people, with 875 stores across 28 US states, and Halle is worth close to $5bn. So what does the man who owes his livelihood to tyres collect? No, not tyres - that would be ridiculous. No, he collects vintage tyre posters. He has a 400-strong selection of lithographs from the past 100 years of the tyre selling business.
 Halle is worth close to $5bn - all thanks to tyres |
"We have a couple Toulouse-Lautrec posters; Leonetto Cappiello, the father of modern advertising. And, of course, we have Marius Rossillon, who went by the name of O'Galup and started doing the big Michelin posters of Bibendum, the big fat guy," he told Forbes.
"It beats collecting old tires, which are big and smell bad."
Bill Gates
Worth: $79.7bn
Collects: Da Vinci, cars and US art
With a net worth of $79.7bn, the Microsoft co-founder is the world's second richest man. Which means even after his noted philanthropic efforts, he has plenty of excess capital to buy collectibles.
And what collectibles.
His 1994 purchase of the Codex Leicester of scientific writings by Leonardo Da Vinci - for $30.8m - is his standout buy, yet American art and cars are his major passions.
 At $30.8m the Codex Leicester is the most valuable book ever auctioned |
In his garage you will find a Porsche 930 Turbo, a Jaguar XJ6 and a Ferrari 348, while in 1998 Gates paid $36m for Winslow Homer's Lost on the Grand Banks - then a record price for an American painting.
Fellow Microsoft founder Paul Allen (net worth $15.7bn) also has a major art collection, though a more classically styled one, with works by Cezanne, Gauguin and Monet. He's also founded museums for aircraft, science fiction and music.
Carlos Slim - the world's richest man
Worth: $80.1bn
Collects: Art - lots of it
Carlos Slim - the world's richest man - leads a busy life. When he isn't being a Mexican telecoms magnate, he's adding to his 66,000-piece collection of fine art.
 Quantity not quality? Slim has a 66,000-piece collection but experts have questioned his taste |
Think Da Vinci, Dali, Picasso, Renoir and Rodin.
In fact, Slim owns the second biggest collection of Rodin sculptures outside France, including iconic pieces such as The Thinker and The Kiss.
And before you complain it's unfair that one of the world's richest men is hoarding all the world's finest art, don't. Because you can see much of Slim's collection for free at his Museo Soumaya in Mexico City.
There you will also be able to view Slim's collection of rare coins, historical documents and religious relics.
With so many pieces on display, it's fair to say the collection could do with some quality control - evidence that even when you have $77.6bn to your name, money can't buy you perfect taste.
Truett Cathy
Worth: $6.3bn
Collects: Classic cars
If you're a chicken eater in the southern states of the US, you'll know about Chick-fil-A - the fast food chain that has been providing chicken sandwiches to the masses since 1946.
The restaurants are a throwback. You won't find any of the 1,600s outlets open on a Sunday, for example.
So it's no surprise that founder Truett Cathy - worth $6bn - is something of a throwback too. The 93-year-old is an avid car collector, and spoke for many a nostalgic automobile buyer when he recently told the Atlanta Journal-Constitution: "I do a lot of things now… that I wished I could do when I was a kid."
As a 13-year-old Cathy pined after his brother-in-law's 1934 Ford Cabriolet, vowing to one day own one. Now he does. It has pride of place in his collection of 70 classic automobiles, which demonstrate Cathy's passion for the unusual as well as the old.
 A 1936 Ford Cabriolet: the vehicle that ignited Cathy's passion for cars |
There's a Gwinnett County fire truck, the Batmobile from 1992's Batman Returns as
