What do the following words have in common?

RUST
IRON 
CLASS 
ARTIST

HINT:
The same two letters will change each word into a new one.

Answers:

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

RUSTIC / IRONIC / CLASSIC / ARTISTIC

Credit: NY Times Crossword (adapted)
September 4, 2011
Dana Delany and Matt Ginsberg
THAT'S DISGUSTING!