how to exit/log out of yahoo mail?

I have an iphone 3gs 1gb. I can log into my yahoo email with no problem, but I don't know how to exit or log out of yahoo mail when I'm done. If I go to my home screen or other aps, then tap on Mail again, it brings up my yahoo email, already signed it. Is there a way to exit out of my email completely? I'd rather have to log in each time I access it, as opposed to having it open all the time.
Thanks for any and all help!