You have been collecting data on the behavior of a particular stock over many years. You notice that every Friday the 13th, the stock drops substantially, only to come back up over the next few weeks. Your conclusion is that superstitious stockholders sell their stock in anticipation of bad luck. What can you do to make use of this information? What effect does your action have? Suppose more people notice the behavior of the stock and react accordingly--what is the effect?
Congratulations to Kaitlin Gossard for providing a reasoned explanation of the likely consequences of the above problem.