there's different types of spying. Tesco etc use their clubcards...you buy sun creams and beachware in tesco, the next day you get emails about travel insurance and foreign currency deals. This is because your information has been sold. this is retained for a long time, once they know what you're buying and when, they can target advertising at you forever. Buying baby stuff and bottles, nappies etc will result in adverts for school uniforms and back-to-school stuff four years down the line