1. What do the following words have in common?

COMB
CAR
FORM
WOMB
CAVE

2. What do the following words have in common?

FLY
GROUND
STICK
HARD
SOFT

HINTS:
1. The same two letters can change each word into a new one.
2. Each clue can be followed by the same word.

Answers:

1. Each word can be followed by AT to make a new word:

COMBAT
CARAT
FORMAT
WOMBAT
CAVEAT

2. Each word can be followed by BALL.

Credits:
1)
LA Times Crossword
Sylvia Burxztyn and Barry Tunick
AT LAST

2)
LA Times Crossword
Sylvia Bursztyn and Barry Tunick
BALL PLAYING