1. The same two letters, in the same order, will change each word below into a new one.

GAS
CENT
HOLY
PHONE

2. The same two letters, in the same order, will change each word into a new one.

ROLE
RENT
TENT

Answers:

1. The letters ME will change each word into a new one:

GAMES
CEMENT
HOMELY
PHONEME

2. The letters PA will change each word into a new one:

PAROLE
PARENT
PATENT

Credits:
#1: Washington Post Crossword
Rich Norris
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#2: NY Times Crossword
August 8, 2007
Donna S. Levin