A Pressing Appointment
Pick any number on a clock face. Starting from 12, spell out that number's English name letter by letter, moving one numeral clockwise per letter. Land somewhere new, adopt that as your next number, and repeat.
Choose 3, for instance: counting T-H-R-E-E from 12 lands you on 5. Counting F-I-V-E from there lands you on 9. Keep going, from any starting number at all, and within three or four rounds you'll reliably land on 1 — and stay there.
Based on a post on Futility Closet.