DUTY to POOP.

Every letter change

1. DUTY → DULY: change letter 3 from T to L.
2. DULY → DULL: change letter 4 from Y to L.
3. DULL → DOLL: change letter 2 from U to O.
4. DOLL → POLL: change letter 1 from D to P.
5. POLL → POOL: change letter 3 from L to O.
6. POOL → POOP: change letter 4 from L to P.