1. What do the following words have in common?

ROB
TOO
ROUGH
DEAR
EFT
BREAD

2. What do the following words have in common?

BAT
CAT
SET
MET
STAR
SHUT

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

Answers:

1. Each word can be either preceded or followed by TH:

THROB
TOOTH
THROUGH
DEARTH
THEFT
BREADTH

2. Each word can be followed by TLE:

BATTLE
CATTLE
SETTLE
METTLE
STARTLE
SHUTTLE

Credit:
#2: LA Times Crossword
       April 16, 2010
       Dan Naddor