As soon as the war broke out, many people became very concerned that the country would be full of German spies. This belief was strengthened when the government decided to intern large numbers of Germans and Austrians. It appeared that secret agents could be anywhere.

The government was also worried by the possible presence of a Fifth Column (people working or spying for the enemy) in the country. As a result, the department responsible for propaganda, the Ministry of Information, began a campaign called 'Careless Talk Costs Lives'.

Many of the posters used in the campaign were designed to be funny. Some showed people having conversations on the bus or in cafés with Hitler listening in the background. Others used cartoon characters such as Popeye to get the message across. The funny nature of the posters helped people to understand their message.

The other famous slogan used on posters was 'Keep Mum - she's not so dumb'. This also encouraged people not to gossip or talk about the war effort in case their conversations were overheard by spies.

The 'Careless Talk Cost Lives' campaign was very successful. It worked because people found the message easy to understand and liked the fact that the posters were funny.