1. What do the following words have in common?

CAR

PRO

WAN

COT 

BUT

2. What do the following words have in common?

RIFT

WART

UNDER

RILL

HINTS:

1. The same three-letter word can be added to each word to make a new six-letter word.

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

Answers:

1. The letters (word) TON can be added to each clue:

CARTON/PROTON/WANTON/COTTON/BUTTON

2. Each word can be preceded by TH:

THRIFT/THWART/THUNDER/THRILL