During wars, it is often very important that information is sent long distances. This information might be about troop movements, or battle plans, and if the enemy read it, they would know exactly what you were going to do. Consequently, in wartime, codes are used to disguise the information in case it falls into enemy hands. Even in the days of the Roman Empire, when letters were sent back by a courier on horseback, his letters would be in code, in case he was captured by the enemy.

One of the first military codes was one that Augustus Caesar used, nearly 2000 years ago. It was a very simple code. The receiver just had to shift the alphabet one position. In other words, "a" was represented by "b," "b" was represented by "c," and so on. Other ciphers can be very very complicated and difficult to break.

Get a piece of paper and work out what this message says - using Augustus's cipher: Bo fbtz djqifs up csfbl!