The five words below can be changed into new ones according to a certain pattern. Can you determine what it is?

SPA
SAT
TEN
HOOP
EXTRA

HINTS:
#1: SPA - a garden tool and a detective
#2: Two letters can be added to each clue word.

Answers:

Each word can be followed by the two letters of a US state postal abbreviation to make a new word:

SPADE
SATIN
TENOR
HOOPLA
EXTRACT

Credit:
NY Times Crossword
February 5, 2012
Charles M. Deber
STATE ANNEXATION