For one-time products, sales rise at the beginning, peak, and approach zero as the number of potential buyers becomes exhausted; if new buyers keep entering the market, the curve will not drop to zero. Infrequently purchased products such as automobiles exhibit replacement cycles dictated by physical wear or obsolescence associated with changing styles, features, and performance. Therefore, sales forecasts must estimate first-time sales and replacement sales separately. With frequently purchased products, such as consumer and industrial nondurables, the number of first-time buyers initially increases and then decreases as fewer buyers are left (assuming a fixed population). Repeat purchases occur soon, providing the product satisfies some buyers. The sales curve eventually falls to a plateau of steady repeat-purchase volume; by this time, the product is no longer a new product.