1. What do the following words have in common?

AVERAGE
STOMACH
TERRAIN
DRASTIC

2. What do the following words have in common?

IT
AGE
OPT
APT
ORE

HINT:
1. It has to do with the first two or three letters of each word.
2. Each word can be preceded by the same two letters.

Answers:

1. AVEerage/ STomach/ TERrain / DRastic 

Address designations for Avenue, Street, Terrace, and Drive 

2.  Each word can be preceded by AD to make a new word:

ADIT / ADAGE / ADOPT / ADAPT / ADORE