1. What do the following three-letter words have in common?

AIR
ALE
ART
END

2. What do the following clues have in common?

Obvious toupee
Harbor vessel
Shoulder heave
Wad of tobacco
Counterfeit coin
Hidden listening device

HINT:
1. The same three letters will change each word into a new one.

Answers:

1.  Each word can be preceded by IMP:

IMPAIR, IMPALE, IMPART, IMPEND

2. In order: A rug, a tug, a shrug, a plug, a slug, and a bug. 

Credit: #2
LA Times Crossword - Feb. 15, 2009
Ed Voile
DIG DUG