Advertising: This is probably the most common revenue model on the Web. Most sites that deliver their content for free make their money through advertising, in the same way that commercials pay for TV shows. A lot of online advertising, like banner ads, is annoying but relatively easy to ignore. This can be a problem, though: some kinds of malware (programs that will harm your computer) pretend to be pop-up ads because they know most people close pop-ups without thinking about it. When you click what you think is the red "Close" button, though, you may actually be leading you to a dangerous Web site or giving the malware program permission to install itself on your computer. When you want to close a pop-up ad, let the cursor hover for a moment over the red "Close" button before clicking: if it looks like a hand instead of an arrow, don't click it -- close the whole tab to get rid of it. (Also, never click any box in a pop-up ad other than the red "Close" button, even if it says "close" or something similar.) Even legitimate ads, however, can use deception. Because many sites that host ads are paid for each time the ad is clicked, sites use a variety of tricks to get users to click through, such as misrepresenting the ad link as something else. The worst example of this is what's called typosquatting, in which advertisers register for Web addresses that are very similar to popular ones, in the hopes that you will type them by mistake; if you misspell Wikipedia.org as "Wikpedia," for instance, you might well land on a site advertising all manner of things; in many cases, typosquatting sites contain pornographic material or malware. The best way to prevent typosquatting is to establish Favorites or Bookmarks for children's preferred sites, so they will not have to type the Web address. In some browsers the Address bar can be removed altogether; while it's not possible in Internet Explorer 7, in Firefox it can be done by clicking on View in the top menu, then selecting Toolbars and unchecking Navigation Toolbar. This is easily undone, however, so it's only likely to be effective with younger children.