A student writes that she became a monopolist in her freshman dorm — hoarding Midol to sell to her dorm-mates at the time each month when dorm-mate had a quite inelastic demand for this product. She also realized that at that time, there is an increasingly inelastic demand for chocolate-chip cookies, so she hoarded and sold those, too. She correctly notes that the two goods are complementary over time — more of both consumed on some days than on others. But I bet that over a short interval, they are substitutes — the satisfaction from one reduces the demand for the other. This illustrates how we need to think about the time dimension of consumer choice. I would also bet that her monopoly doesn’t last long. Anybody can bring the two products to the dorm and sell them — there are few barriers to entry. A better description is that she’s an innovating entrepreneur in what inherently will be a competitive industry.