What do the following words have in common?

VET
MET
UGH
GENT
UPON 
PIOUS
WARD
MEDIAN

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

Answer:

Each word can be preceded by CO to make a new word:

COVET
COMET
COUGH
COGENT
COUPON
COPIOUS
COWARD
COMEDIAN

Credit: Will Shortz
NPR Weekend Edition Sunday Puzzle
July 8, 2007 - COPIOUS ENJOYMENT