One of the two words economists use most; the other is supply. These are the twin driving forces of the market economy. Demand is not just about measuring what people want; for economists, it refers to the amount of a good or service that people are both willing and able to buy. The demand curve measures the relationship between the price of a good and the amount of it demanded. Usually, as the price rises, fewer people are willing and able to buy it; in other words, demand falls (but see Giffen goods, normal goods and inferior goods). When demand changes, economists explain this in one of two ways. A movement along the demand curve occurs when a price change alters the quantity demanded; but if the price were to go back to where it was before, so would the amount demanded. A shift in the demand curve occurs when the amount demanded would be different from what it was previously at any chosen price, for example, if there is no change in the market price, but demand rises or falls. The slope of the demand curve indicates the elasticity of demand. For approaches to modeling demand see revealed preference. Policymakers seek to manipulate aggregate demand to keep the economy growing as fast as is possible without pushing up inflation. Keynesians try to manage demand through fiscal policy; monetarists prefer to use the money supply. Neither approach has been especially successful in practice, particularly when attempting to manage short-term demand through fine tuning.