What do the following words have in common?

FED
LOSE
CASE
CLOD
PROD
MOTH
POND
STATE
BOLDER

HINT:
The same letter, appropriately placed, will change each word into a new one.

Answer:

Inserting the letter U into each word will create new words:

FED - FEUD
LOSE - LOUSE
CASE - CAUSE
CLOD - CLOUD
PROD - PROUD
MOTH - MOUTH
POND - POUND
STATE - STATUE
BOLDER - BOULDER