For safety
Disabling automatic sign-in
If you're being automatically signed in to your Google Account, you may have enabled either a password-storing feature in your browser, or the Google Accounts option to "Stay signed in."
Google Accounts
First check if the Google Accounts "Stay signed in" option is actually enabled. Visit the Google Accounts homepage and verify if the checkbox above the Sign In button is selected. If it's selected, delete your browser's cookies. If you're using Mozilla Firefox version 1.5 or later:
Go to the Tools menu.
Select Options.
Select the Privacy icon.
Click the Cookies tab and the Clear Cookies Now button.
Click OK to exit.
Each time you sign in to your Google Account, make sure the "Stay signed in" option isn't selected.
If the problem persists, you may want to clear your browser's cache and cookies. Please note that while deleting your cookies may resolve the problem, it will also remove your saved settings for sites you've previously visited.
source : http://support.google.com/
