he proceeded to lecture me-hovering there with that horrid needle glistening an inch from my nose-on how prized this poison was ; how the czar of russia kept a stash of it in the royal safe ; how it was valued by the ancients ( 'they say it was what really killed cleopatra ' ) ; how it was the method of choice of assassins , preferred because it was so slow-acting , allowing the perpetrator to be miles away by the time the victim 's heart exploded in his chest .