Supply and Demand
Supply and demand is one of the most fundamental, central concepts of economics, so it’s an important one to understand. Here are some resources to explore and learn about supply and demand:
http://hadm.sph.sc.edu/courses/econ/sd/sd.html
http://www.socialstudiesforkids.com/articles/economics/supplyanddemand1.htm
http://www.investopedia.com/university/economics/economics3.asp