he simple circular flow is, therefore, adjusted to take into account withdrawals and injections. Households may choose to save (S) some of their income (Y) rather than spend it (C), and this reduces the circular flow of income. Marginal decisions to save reduce the flow of income in the economy because saving is a withdrawal out of the circular flow. However, firms also purchase capital goods, such as machinery, from other firms, and this spending is an injection into the circular flow. This process, called investment (I), occurs because existing machinery wears out and because firms may wish to increase their capacity to produce.