Substitution ciphers are not safe enough to use for important messages. Substitution ciphers can be broken by an idea called "frequency analysis". Some letters are more common than others in English sentences: "E" is the most common, then "T", then "A", and so on. A message that has been changed by a substitution cipher will have different common letters, but this gives a hint about the rule. The most common letters in the changed message are likely to be the most common letters in English. Breaking "cryptograms" (messages hidden with a substitution cipher) is a common puzzle often found in newspapers.