The crib
A guessed piece of message is called a crib. It does not have to be much: a station that always signs off the same way, a report that always begins with the weather, a letter that starts Dear Sir.
One crib gives you a piece of the key, and a piece of the key usually gives you the rest.
At Bletchley Park this was the main way in. German weather stations sent forecasts every morning in the same shape, and the word WETTER turned up where it was expected. Predictability, not weakness in the machine.
Making a crib on purpose
If guessing a word is that powerful, you can arrange for one. The Americans suspected the Japanese code word AF meant Midway, so they had Midway broadcast in clear that its water plant had broken. Japan duly reported that AF was short of water, and the suspicion became a certainty.
The British did the reverse with gardening: mines were dropped in a chosen patch of sea so that the Germans would broadcast that patch's name, and that name became a crib.
Which is why real ciphers now avoid predictable openings entirely. No Dear Sir, no fixed greeting, and no message beginning with the same word twice. The cipher is only as strong as the most guessable thing inside it.
Keyspace The one-time padRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.