1. What do the following words have in common?

MOON
COAL
ACID

2. What do the following words have in common?

BIN
CAT
TAIL
WHITE

HINTS:
1)
#1: HOSE could be added to the list.
#2: FIST could be added to the list.
#3; The same letter can be inserted into each word in the same place to make a new word.

2)
#1: SLED could be added to the list.
#2: The same three-letter word can precede each clue to make a new word.

Answers:

1. The letter R can be inserted into each word as the third letter to make new words:

MORON
CORAL
ACRID
(HORSE)
(FIRST)

2. The word BOB can precede each word to form a new one:

BOBBIN
BOBCAT
BOBTAIL
BOBWHITE
(BOBSLED)