1. What do the following words have in common?

WAG
BAT
BAR
TEN

2. What do the following words have in common?

ON
RAT
ACT


HINTS:
1. The same two letters, in the same position, will change each word into a new one.
2. The same three letters, in the same position,  will change each word into a new one.

Answers:

1. Adding the letters ON at the end of each word will give

WAGON / BATON / BARON / TENON

2. Adding the letters ION at the end of each word will give

ONION / RATION / ACTION