Office challenge: Why does Word replace ADN with and and how can you make it stop?

This week, share your Word skills by exposing why Word replaces the three-letter acronym ADN with the word "and."

A frustrated user calls you with what he believes to be a serious bug in Word. Every time he types the acronym ADN, Word replaces it with the word and. Can you help this user train Word to stop automatically replacing ADN with and?