Modern money -- typical, paper-based fiat currency -- typically does not have any inherent value; its only value is in the head of whomever is doing the buying and selling. Outside of that frame of reference, it's usually pretty much worthless. Lots and lots of items have been used as currency throughout history, although before the past 100 years or so, commodity currency was the standard, meaning it was currency that had worth in and of itself. This could include prized commodity products like tobacco, tea, cattle, horses, gold or silver.

Commodity currency has a long history, and came into popularity as early human civilizations started to realize the challenges presented by the barter system or gift economy. As social populations became increasingly complex, the potential number of requisite swaps needed to accomplish successful trades made the process increasingly complicated and unpractical. The concept of money alleviated this problem by providing proxies that everyone could agree on as worth trading for.

Money, as a thing to exchange for goods and services, has had stranger forms than, say, trading horses. Take cowries, for example. Cowries are small, marine snails that live in the warm waters off the West Indies, along America's Pacific coast and in Asian tropical waters, and their shells were once used as money in Africa and India. The shells are glossy and oval-like, with toothed edges. British merchants collected cowry shells in the East Indies and then used them to barter with locals. The cowries used for money were yellow or white and measured about 1 inch (2.5 centimeters) long. But there are also more colorful cowries, and those were used to make jewelry.

Today, another interesting example of a kind of money might be the currency used to buy things in Massively Multiplayer Online games. Players of games such as "World of Warcraft" use actual money to buy any number of virtual goods helpful to their game-paying experience. Virtual currency also is making its way in the world of games for social networking sites.