I understand that there are plenty of options and tools to stop cookies from being set, or to delete them automatically, and maybe I don't understand some fundamental of how cookies function, but it seems like the current problem in privacy is sites tracking what I do when I am not on their site. As I understand it, this sounds like a site can retrieve cookie information set by another site that it isn't even associated with. Is this correct? Is that somehow a necessary function of websites? Is there a way to prevent that? I've always assumed that blocking the cookie set by an ad company will block the company from tracking me, but the tracking by facebook that everyone is talking about seems different. I can't find reference to a particular cookie that I could just block and forget about. It sounds like they're hoovering up everyone else's cookies, and that seems awfully strange.