1. A country at war must be able to convey information to military personnel. That is always a challenge.

2. The information’s importance often requires it to be transmitted secretly.

3. Military strategists use codes for these transmissions because they baffle the enemy.

4. They say that “invisible ink,” which cannot be seen until the paper is heated, was once a popular way to communicate secretly.

5. Lemon juice and vinegar are good choices for invisible ink because you can’t see them unless they are burned.

6. During World War II, U.S. government code specialists hired Navajo Indians because it is a difficult and little-studied language.

7. In early code writing, it involved substituting letters throughout the message.

8. These cryptograms are no longer used to transmit messages because they are too simple.

9. The Nazis’ Enigma code was extremely difficult to crack. This was an enormous problem for the Allied forces.

10. Alan Turing’s mathematical genius saved the day. He was a British civil servant who finally solved the Enigma code.