A Rotting Word:
Becomes another English word when rot-x is applied, where 'x' is the length of the word itself.
(NB 'rot-x' is shorthand for a Caesar shift of 'x' positions onward through the alphabet...)
Like follows:
admin -- rot-5 --> firns
box -- rot-3 --> era
dawn -- rot-4 --> hear
elm -- rot-3 --> hop
fang -- rot-4 --> jerk
fusion -- rot-6 --> layout
lean -- rot-4 --> pier
opal -- rot-4 --> step
open -- rot-4 --> stir
pawn -- rot-4 --> tear
Note that while most of these are common English words, 'firns' refers to snow that has gone through multiple thaw-and-refreeze cycles.
As for the Not Rotting Words:
All of these do still form real words when a Caesar shift is applied, but in these cases the value of the shift is not equal to the length of the word itself:
job -- rot-12 --> van
tube -- rot-14 --> hips
sleep -- rot-9 --> bunny
river --rot-9 --> arena
stun -- rot-11 --> defy
bomb -- rot-6 --> hush
wheel -- rot-7 --> dolls
spots -- rot-11 --> dazed
hotel -- rot-7 --> ovals
bug -- rot-6 --> ham