How did the habits of Enigma operators help break their own codes?

Each message sent on Enigma, the German military’s cipher machine in World War II, began with a short setting the operator was supposed to pick at random. Many did not: they used neighboring keys, what was left over from the last message, or a girlfriend’s name.

At Bletchley Park, Britain’s wartime codebreaking center, these guessable settings were called “cillies.” Spotting one helped codebreakers work out that day’s Enigma settings, and with those they could read every message sent on the network that day.