Supply refers to the number of goods that are available. Demand refers to how many people want those goods. When the supply of a product ascends, the price of a product descends, and demand for the product can rise because it costs less. At some point, too much of a demand for the product will cause the supply to lessen. A fundamental economic principle that when supply exceeds demand for a good or service, prices fall. When demand surpasses supply, prices tend to rise. There is a flip-side relationship between the supplies and prices of goods and services when demand is not changed.