Suppose Janet gives haircuts on Saturdays to make extra money. She is the only person in town cutting hair on Saturdays, so she has some market power. Assume that she does not incur fixed costs and that the only significant variable cost to Janet in giving haircuts is her time. As she gives more haircuts, Janet must increasingly forgo other valuable Saturday activities. For example, if she gives one haircut, she forgoes reading the paper after breakfast. If she gives two haircuts, she gives up reading the paper, sleeping an extra half-hour, and so on.