WHEN to POOP.

Every letter change

1. WHEN → WHEW: change letter 4 from N to W.
2. WHEW → CHEW: change letter 1 from W to C.
3. CHEW → CHOW: change letter 3 from E to O.
4. CHOW → CHOP: change letter 4 from W to P.
5. CHOP → COOP: change letter 2 from H to O.
6. COOP → POOP: change letter 1 from C to P.