How can I communicate a password to a recipent solely by email without using plain text?

The idea is that you would attach a password encrypted ZIP file; but anybody else reading the email would not know the password. If I ask him to answer a question, the answer being the password, it would have to be one that even his wife would not know the answer to. But, it has to be simple enough so that he would get it right without ambiguity. I want to discount "easy" answers like calling him and communicating a password by phone; or in person. Any clever ideas?

How can I communicate a password to a recipent solely by email without using plain text?
Encryption. Sound like a job for PGP http://www.pgp.com/





You'll both need security certifiactes to verify sender and recipient.