The time-preference principle, it must be emphasized, applies only to comparisons involving the consumption of a single good. Apparent exceptions to the principle, when examined more closely, generally involve comparisons between two related but different goods. For instance, a woman may defer drinking a bottle of wine in order to allow the vintage to mature. Yet it is clear that aged wine is a different and more valuable good than the younger wine, so that her choice does not violate the time-preference principle. Over a briefer period (which might be omitted from a less detailed analysis), she may defer consumption simply because she is not yet thirsty. In the latter instance, however, the context in which the wine is consumed changes in such a way as to render it a technically distinct good—just as ice in the summer provides services that ice in the winter cannot and is therefore a distinct good (p. 4.4:28).