The letter 's' appears in several possible words matching our pattern like "fast", "last", and "cast". It's also one of the most common consonants in English, making it a high-probability guess. If 's' is the first letter, it would give us "s_a_t" which could be "spat" or other words, and if it's the third letter it would give us "__ast" which could be "fast", "last", "cast", etc.