1. What do the following words have in common?

TEST
DUCT
TRACT

2. What do the following words have in common

AIR
ART
ACT
EACH

HINTS:
1. Think of prefixes.
2. The same three letters will change each word into a new one.

Answers:

1. Each word can be preceded by both PRO and CON to make valid words:

PROTEST/CONTEST
PRODUCT/CONDUCT 
PROTRACT/CONTRACT

 2. Each word can be preceded by IMP to form a new word: 

IMPAIR
IMPART
IMPACT
IMPEACH