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Problems with Cookies
People often share machines -- Any machine that is used in a public area, and many machines used in an office environment or at home, are shared by multiple people. Let's say that you use a public machine (in a library, for example) to purchase something from an on-line store. The store will leave a cookie on the machine, and someone could later try to purchase something from the store using your account. Stores usually post large warnings about this problem, and that is why. Even so, mistakes can happen. For example, I had once used my wife's machine to purchase something from Amazon. Later, she visited Amazon and clicked the "one-click" button, not realizing that it really does allow the purchase of a book in exactly one click. 
On something like a Windows NT machine or a UNIX machine that uses accounts properly, this is not a problem. The accounts separate all of the users' cookies. Accounts are much more relaxed in other operating systems, and it is a problem. 
If you try the example above on a public machine (in a library or school, for example), and if other people using the machine have visited How Stuff Works, then the history URL may show a very long list of files. 


Cookies get erased -- If you have a problem with your browser and call tech support, probably the first thing that tech support will ask you to do is to erase all of the temporary Internet files on your machine. When you do that you lose all of your cookie files. Now when you visit a site again, that site will think you are a new user and assign you a new cookie. This tends to skew the site's record of new versus return visitors, and it also can make it hard for you to recover previously stored preferences. This is why sites ask you to register in some cases -- if you register with a user name and a password, you can re-login even if you lose your cookie file and restore your preferences. If preference values are stored directly on the machine (as in the MSN weather example above), then recovery is impossible. That is why many sites now store all user information in a central database and store only an ID value on the user's machine. 
If you erase your cookie file for How Stuff Works and then revisit the history URL in the previous section, you will find that How Stuff Works has no history for you. The site has to create a new ID and cookie file for you, and that new ID has no data stored against it in the database. [Also note that the How Stuff Works Registration System <http://www.howstuffworks.com/register.htm> allows you to reset your history list whenever you like.] 


Multiple machines - People often use more than one machine during the day. For example, I have a machine in the office, a machine at home and a laptop for the road. Unless the site is specifically engineered to solve the problem, I will have three unique cookie files on all three machines. Any site that I visit from all three machines will track me as three separate users. It can be annoying to set preferences three times. Again, a site that allows registration and stores preferences centrally may make it easy for me to have the same account on three machines, but the site developers must plan for this when designing the site. 
If you visit the history URL demonstrated in the previous section from one machine and then try it again from another, you will find that your history lists are different. This is because the server created two IDs for you on the two machines. There are probably not any easy solutions to these problems, short of asking users to register and storing everything in a central database.