1. What do the following words have in common?

HUM
SNARE
KETTLE
EAR

2. What do the following words have in common?

CORD
FLAG
DRAG

HINTS:
1. Each word can be followed by the same four-letter word.
2. The same two letters can be added to each word.

Answers:

1. Each word can be followed by DRUM. 

2. Each word can be followed by ON:

CORDON
FLAGON
DRAGON

Credit: Boston Globe Crosswored
Henry Hook
ADDING ON