Ubuntu provides a really great, simple way to get around this problem.
You will most likely be prompted for your password.
This is a really great feature.
Let Somebody Use Your Computer Without Logging Off in Ubuntu
How many times have you been right in the middle of doing a thousand things on your computer and your friend or significant other wants to use the computer to look something up? You are worried that they are going to close the entire browser session when they are done, cause they don't know what they are doing and won't just close the tab.
