有人说金钱能推动世界运转。作为交易中不可缺少的一环，“钱”这一概念将会怎样改变？从以物易物式的古老经济制度到如今使用现金、信用卡、甚至是网络虚拟货币等支付手段，人们对金钱的定义和理解在不断地更新。

How do you pay for things in a shop? Perhaps you like the tangible reliability of hard cash? Maybe the financial flexibility of a credit card suits you better? Or perhaps you prefer the simple convenience of a smartphone?

Whatever you use today, experts believe all these methods could soon become outdated. Instead, we will use our bodies: our eyes, our fingerprints, even our mere presence in the store. In fact it's happening already. Amazon are trialling stores which have no checkouts, where technology tracks the items you've taken from the shelves and deducts the total from your account when you leave the shop.

French supermarket Monoprix takes a different path: you choose your groceries and leave them with a human cashier. You then leave the shop while the cashier tallies up your bill, charges your account, and organises delivery to your home.

Amir Sajed, chief executive of Barclaycard, told the BBC that such new developments spell the end of the plastic credit card. Instead, wearable items such as rings, bracelets and keychains will carry chips that allow shoppers to "seamlessly shop, going between the web, an app or in store," he says.

And while all the above payment methods are underpinned by accounts held in traditional currencies, let's not forget the rise of alternatives such as Litecoin. Such virtual currencies can rise in value very quickly, but are also susceptible to crashes and threats from hackers. Who knows, perhaps something totally new will take off that changes money as we know it? One such possibility is explored in the movie In Time. It imagines a futuristic society in which the currency is time itself, where people trade the amount of time they have left to live.

Or perhaps we'd do better to wind back the clock to the simpler financial world of the barter economy. While the term conjures images of sacks of grain and herds of sheep being exchanged in ancient times, there are signs that bartering is making a comeback in today's world of modern technology. Startup Let's Barter India has developed an app which facilitates the exchange of goods, and already has around 100,000 members.

Maybe the only thing we know for certain is that money will keep evolving.