1. What do the following words have in common?

BE
US
PI
CAT
CAP

2. Each letter shown below can begin two related words. The four word pairs thus formed are related in the same way. What are the new words and how are they related?

B
G
P
T

Answers:

1. The letters ER can be added to each word to make new ones:

BEER
USER
PIER
CATER
CAPER 

2. Each letter can be followed by both IN and OUT to make valid words:

BIN    BOUT
GIN   GOUT
PIN    POUT
TIN   TOUT 

Credit #2: 
Matt Ginsberg
REVOLVING DOORS