Prison Yolk

Giganews Newsgroups
Subject: Prison Yolk
Posted by:  J J Levin (jjlev…
Date: Sun, 4 Jan 2009

From Jeffrey Archer's new book, PRISONER OF BIRTH (a wonderful thriller, by
the way):

The setting: the protagonist, Danny, was eating 2 boiled eggs for breakfast
and was interrupted repeatedly by the phone.

Then  follows:

"Danny looked down at a prison yolk. 'Molly, could you boil me another
couple of eggs?'. "

What is "prison yolk" ??