I found a really interesting story in Johan Norberg's Open: The Story of Human Progress.

. . .a friend sent me a message from his children's school about a problem with snack boxes. Apparently, the children had started trading food with one another. And rice cakes in the boxes created bigger problems than anything else because children at school had started using them to pay for other goods and even to buy help and services. The school wanted the parents' help to stop the kids from being free traders. The children had realized that by bartering they could get something to eat that they preferred to what they already had, so after an exchange both thought they had a better snack box than before. They even developed a medium of exchange -- rice cakes -- that they realized they could use to extend the market.

See, the market emerges.