1. What do the following words have in common?

ARROW
EMBER
IRE
ORES
UTTER

2. What do the following words have in common?

BEAR
POWER
BOY
CROW

HINT:
2. The same letter, when properly placed, will change each word into a new one.

Answers:

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

MARROW
MEMBER
MIRE
MORES
MUTTER

Credit: GAMES Magazine - Sept. 1982
H. Ferguson 

2. The letter D can be added to or inserted into each word to form new one:

BEARD
POWDER
BODY
CROWD 

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
ADDD