Problems with sessions

Hi people, I need your help: I don't understand sessions very well. I'm programming a typical login system. The user input his user name and password, and I check this information in a MySQL database. If the user and password exists, I do the following: session_start(); session_register("authenticated"); $authenticated="yes"; Then, the user has a menu (made with Flash) and can go to diferents pages (each page is a diferent php file). In each one of those pages I check that the user is authenticated: if($authenticated!="yes") { echo "Login error"; } else { ... But $authenticated never has the value yes. I suppose that this is because the $authenticated doesn't conserve the value between pages, but this the characteristic of a session variable. Or not? Ahead of time, thank you very much, Tom.