... effect tends to make the consumer buy more pizza and more Pepsi. Yet, at the same time, consumption of Pepsi has become less expensive rela- tive to consumption of pizza. This substitution effect tends ... the law of demand and buy more of a good when the price rises. To see how this can happen, consider Fig- ure 2 1-1 2. In this example, the consumer buys two goods—meat and potatoes. Ini- tially, ... fact a Giffen good during the Irish potato famine of the nineteenth century. Potatoes were such a large part of peo- ple’s diet that when the price of potatoes rose, it had a large income effect....