1. What do the following words have in common?

BAT
MEND
MUTE
PARE
PASS
PLAIN

2. What do the following words have in common?

INTER
LACE
RENT
ROLE
TENT

HINTS:
1. The same three 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 COM to form a new word:

COMBAT
COMMEND
COMMUTE
COMPARE
COMPASS
COMPLAIN

2. Each word can be preceded by PA to form a new word:

PAINTER
PALACE
PARENT
PAROLE
PATENT

Credit: Both puzzles: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
#1:  COM ON
#2:  POP IN