The eight words below can be arranged into two groups of four according to a certain pattern. Can you determine the pattern?

INK

EAR

HEAL

READ

FOUR

STEAL

UNDER

ROUGH

 

Answer:

 

The two groups are determined by whether the letters TH can either precede or follow each word to make a new one:

 

THINK                  EARTH

THREAD              HEALTH

THUNDER           FOURTH

THROUGH          STEALTH