1. What do the following words have in common?

TEST
TIED
SHOW
STING
DYING

2. What do the following words have in common?

AD
GAR
POST
VOLT
BOND

HINT:
1. The same three letters can be inserted into each word.

Answers:

1. The letters (word) ALL can be inserted into each word to make a new one:

TALLEST
TALLIED
SHALLOW
STALLING
DALLYING 

2. The letters (word) AGE can be added at the end of each word to create a new one:

ADAGE
GARAGE
POSTAGE
VOLTAGE
BONDAGE

Credit: Both puzzles - LA Times Crossword
Sylvia Bursztyn and Barry Tunick
 #1: ALL ABOARD and ALL ABOARD II 
#2:  ADDED MATURITY