The same change can be made to each of the words below to change them into new ones. Can you determine what it is?

SUE
FINE
POLE
WHEN
GRAVY
SUBTLE

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

Answer:

The letters IT can be inserted into each word to change it into a new one:

SUE - SUITE
FINE - FINITE
POLE - POLITE
WHEN - WHITEN
GRAVY - GRAVITY
SUBTLE - SUBTITLE 

Credit: Will Shortz
NPR Weekend Edition Sunday Puzzle
June 24, 2007 - IT'S "IT"