Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic satirizes the idea of using economic incentives to motivate children. A mother tells her daughter Sally that they're going to "economically incentivize" her to stop playing video games by paying ten dollars for each book report she produces. The comic then shows three possible outcomes: a "Bad Kid" who responds with indifferent "Meh. Whatever," a "Good Kid" who eagerly agrees, and a "Future Economist."

The future economist is shown outside the house with a sign reading "Will buy book reports, $7 each!" Rather than doing the work herself, she has found a way to exploit the system through arbitrage -- buying book reports from other kids at seven dollars and selling them to her parents for ten, pocketing a three-dollar profit on each transaction without ever reading a book. The joke is that the parents' attempt to use market incentives backfired because a truly economically-minded child will optimize for profit rather than learning, finding the most efficient path to the reward rather than the intended behavior.