1. What do the following words have in common?

YELL
FELL
PILL
WIND
CALL

2. What do the following words have in common?

STICKING
BREAKING
FREEZING
BOILING

Answers:

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

YELLOW
FELLOW
PILLOW
WINDOW
CALLOW 

2. Each word can be followed by POINT.