Riddle #4 and #5 and #6

4
I've got teeth, but I can't bite.
I am the guardian of your house.
You won't leave home, or come home, without me.
I'm not loyal, leave me behind, and I'll let the next man enter.

What am I?
5

What remains dark when all else is light?
6
A man worked at a high security institution. The man tried to log into his computer and the computer denied the password. He then remembered that the passwords to the computers were reset every month for security reasons. He called his boss for his new password.

The man said, "Boss, my old password is out of date."

The boss said, "Yes, it is. The new password is different, but if you listen closely you will be able to figure out the new one. Your new password has the same amount of letters as the old one, and four of the letters are the same."

The man then logged into his computer with no trouble. What was the new password? What was his old?


answer:
The man's old password was "out of date".
His new password was "different".