A typical business cycle plays out in the following way. Starting from some point in the business cycle, some group (consumers, government, purchasers of exports, etc.) decides for some reason to have a sustained increase in their spending. This may come as a surprise to producers, who initially experience negative inventory investment as their sales have unexpectedly exceeded their production. Now their inventories are too low, for two reasons: (1) Inventories have accidentally gone down, and (2) the optimal level of inventories—what producers want to have on hand—has gone up because sustained customer demand has gone up and there is increased danger of temporary stock-outs. In order to build inventories up to an appropriate level, firms engage in positive intended inventory investment. This positive flow of intended inventory investment continues until the target level of inventories is reached. During this time, the economy is in a boom both due to the original sustained increase in spending and due to the positive flow of intended inventory investment.