There are two important differences between the Amazon.com scenario and what happens when DoubleClick serves a banner ad. First, the visitor usually does not know that the banner ad is coming from DoubleClick (and thus, does not know that the DoubleClick server could be writing a cookie to the client computer). Second, DoubleClick serves ads through Web sites owned by thousands of companies. AS a visitor moves from one Web site to another, that visitor’s computer can collect many DoubleClick cookie. The DoubleClick server can read all of its own cookie, gathering information from each one about which ads were server and the sites through which they were served. Thus, DoubleClick can compile a tremendous amount of information about a user’s action on the Web