What do the following words have in common?

1. BARB
2. MOTH
3. NUMB
4. CORN
5. FLOW
6. MIST

HINT:
The same two letters, in the same position, will change each word into a new one.

Answer:

Adding the letters ER to each word will change each word into a new one:

1. BARBER
2. MOTHER
3. NUMBER
4. CORNER
5. FLOWER
6. MISTER

Credit: LA Times Crossword
Sylvia Burzstyn and Barry Tunick
OUST-ER  (#1-3)