The ducks in St James's Park are so used to being fed bread by secret 
agents meeting clandestinely that they have developed their own Pavlovian 
reaction. Put a St James's Park duck in a laboratory cage and show it a 
picture of two men -- one usually wearing a coat with a fur collar, the 
other something sombre with a scarf -- and it'll look up expectantly. 
        -- (Terry Pratchett & Neil Gaiman, Good Omens)