The same two letters, in the same, consecutive order, can change each word below into a new one. What are the letters and what are the new words?
 
ME
PASTE
GRACE
ESTATE

HINT:
The two letters have to be inserted into each clue word.

Answers:

The letters are IM:

MIME
PASTIME
GRIMACE
ESTIMATE

Credit:
Richard Silvestri
I'M IN!