Commodities

Definitions:
1. A commodity is a good for which there is demand, but which is supplied without qualitative differentiation across a market. (Wikipedia)
2. A physical substance, such as food, grains, and metals, which is interchangeable with another product of the same type, and which investors buy or sell, usually through futures contracts. The priceof the commodity is subject to supply and demand. Risk is actually the reason exchange trading of the basic agricultural products began. For example, a farmer risks the cost of producing a product ready for market at sometime in the futurebecause he doesn't know what the selling price will be.(http://www.investorwords.com/975/commodity.html)
Explanation:

In my own words a commodity is something that is in pretty high demand that everyone wants that hasn't been really prices or anything just what you think it is worth. Your natural response pretty much.

Metaphor
If someone has a pair of keys and you want them you have to figure out what those keys are worth to you and so I might say they are worth 20 bananas. Then I will tell that person how much I would pay for it. Then he would tell me how much he thinks it is worth and then we would bargain.