1. What do the following three words have in common, other than starting with the same letter?

RED
RUG
RINK

2. What do the following words have in common?

STEP
POSE
LATE
SUPER
RUBLE
CORAL
BELOW
ROSTER

Answers:

1. Each word can be preceded by SH to make a new word:

SHRED
SHRUG
SHRINK

2. Doubling one of the interior letters in each word creates a new word:

STEEP
POSSE
LATTE
SUPPER
RUBBLE
CORRAL
BELLOW
ROOSTER

Credits:
#1: Nancy Salomon
#2: Joseph Appel