1. What do the following words have in common?

PIER
GENT
PIOUS
UPON
PING

2. What do the following four-letter words have in common?

BALL
JACK
PLAN
MALL
CARP
TICK

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

Answers:

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

COPIER
COGENT
COPIOUS 
COUPON
COPING

2. Each word can be followed by ET to make a new word:

BALLET
JACKET
PLANET
MALLET
CARPET
TICKET