1. What do the following words have in common?

BEG
BULLET
CODE
PUFF
ROB

2. What do the following words have in common?

PAY
EGG 
SWEET
STEAM
FRENCH
CRESCENT

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

Answers:

1. Adding IN to each word will change it into a new one:

BEGIN
BULLETIN
CODEIN
PUFFIN
ROBIN 

2. Each word can be followed by ROLL.

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
ROLL PLAYING