1. What do the following words have in common?

LET
PAR
DUE
SIDE
LIME

2. What do the following words have in common?

READ
RUSH
EATER
OUGHT
ROUGH

HINTS:
1. Each word can be preceded by the same three letters.
2. Each word can be preceded by the same two letters.

Answers:

1. Each word can be preceded by SUB to give a new word:

SUBLET
SUBPAR
SUBDUE
SUBSIDE
SUBLIME

2. Each word can be preceded by TH to give a new word:

THREAD
THRUSH
THEATER
THOUGHT
THROUGH