Sites can't see the existence of other sites' cookies. They can, however, request resources from other sites. Those requests, in turn, would send third-party co

Sites can't see the existence of other sites' cookies. They can, however, request resources from other sites. Those requests, in turn, would send third-party cookies, if any, to the third-party server _and_ save new third-party cookies in response. "Tracking beacons" abuse this behavior to correlate user behavior across many web sites.