Retailers have traditionally opened early on "Black Friday," the Friday after Thanksgiving. Indeed, many have opened even earlier, on Thanksgiving night. If just one retailer did this, it would gain an advantage, but when they all do this, they just spread their sales revenue over two days instead of one and they have higher costs because they are open more hours. As a result, retailers would all be better off if they all were closed on Thanksgiving. What does this Back Friday experience illustrate?