We usually think about demand in the two dimensional way that we are restricted to visualizing it in with a graph. That is, the amount demanded is a function of price. In actuality there are several other factors which determine demand that are embedded in those graphs but not visualized. For instance, we know that the level of income influences demand as does the availability and price of substitutes and something called tastes and preferences. For instance, if you hate carrots, lowering the price is not likely to induce more quantity demanded, though we might show the demand curve for carrots for a group (like people in the U.S.) as a function of the price.
