What do the following words have in common?

EMPIRE
FIBER
BORING
GRAY
FLAT
TESTING

HINT:
The same letter can be exchanged with one letter in each word to create new words.

Answer:

The letter X can be exchanged for one letter in each word to create new words:

EXPIRE
FIXER
BOXING
XRAY
FLAX
TEXTING

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
X - CHANGE