Some sites refer to a zero-sized image hosted on a different host to "spy" users habits as the request has information stored on cookies. This test needs two different hosts to be used. To store a cookie from the other host visit this link:

http://it.gnu.org/~gscrivano/gnuzilla/zero_len.php


After you stored the cookie with this link, refresh the current page that refers to a zero-sized image on the other host. When the image is requested the browser will send back the previous stored cookie.