1.What do the following words have in common?

TAN
ORB
PERM
VISOR
CIRCUS

2. What do the following words have in common?

PIT
AID
TUB
PAST
PARK

HINTS:
1.The same two letters will change each word into a new one.
2. The same letter will change each word into a new one.

Answers:

1. Inserting or adding the letters IT to each word will change it to a new one.

TITAN
ORBIT
PERMIT
VISITOR 
CIRCUITS

2. Adding an A to the end of each word creates a new one:

PITA
AIDA
TUBA
PASTA
PARKA