The Muffin Tops
Elaine and her old boss find that selling just the tops of muffins is more profitable than selling the whole muffin (which consists of the top + the stump). So are the top and the stump complements or substitutes? Neither--the stumps are an economic bad, which reduce utility. Evidence for this is found in the fact that homeless people won't eat the stumps that the muffin-top restaurant throws away, unless they come with the tops as compensation.