Leopard sports a brand new set of parental controls. This is a brand new system preference. You can set basically anything from when a specific account can access the internet, where they can go, who they can chat with, what words they can say, who they can get email from, etc. You can set time limits as well. Parental controls also limits what they can do with the computer itself such as what applications they can access, whether or not they can install any new applications, etc. All settings are account specific and are controlled by an administrator account.
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