NPR Weekend Edition Sunday Puzzle
March 25, 2018
Will Shortz

The following words share an unusual quality. Can you determine what it is?

PRIEST
POSTER
STRING
SKIING
RATE
SUDS

HINT:
It has to do with double letters.

Answer:

Each word can be changed into a new one by inserting a pair of double letters - or two consecutive pairs of double letters - in the case of the last two words:

PRIMMEST
POLLSTER
STEERING
SKIPPING
ROOMMATE
SUCCEEDS