What do the following words have in common?

RAIL
RIVER
DIARY
COSMIC
DONATOR

HINTS:
#1: MARK, ROCK, and TICK could be added to the list.
#2: The same two letters will change each word into a new one.

Answers:

The letters ET can be inserted into (or added to) each word to form a new one:

RETAIL
RIVETER
DIETARY
COSMETIC
DETONATOR
(MARKET/ROCKET)

Credit: NY Times Crossword
February 28, 1999
Dave Tuller
ALIEN ABDUCTIONS