Warning: Page has Expired

I've setup a members directory in mysql that is searchable via different field and I limit the number of records. I use session variables to remember the number of records to display, what the starting record is, what the search string and search fields are. At anytime a user can click on an id number to do a Select statement that queries for that single row of data. Afterward, if the user hits their browsers back button I get: "Warning: Page has Expired" I've noticed allot of sites just ask the user to not use the back button. I'm curious if this is caused by an error in my code or because I used session variables that might have changed in the last MySQL select query. So that when you hit back, the browser is using some sort of cached data while still loading the php code? I'm very confused at this point. Is there anything I can do to prevent the "Expired Page" warning, rather than expecting my viewers to learn to do without it. Keith Spiller webtech@centerstone.org