1. What do the following words have in common?

BAD
BET
BOON
MET
TEST

2. What do the following words have in common?

BAR
BIN
MAN
SIN
TWIN

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

Answers:

1. Each word can be changed into a new one by inserting the letters (word) ALL:

BALLAD
BALLET
BALLOON
MALLET
TALLEST 

2. The letters GE can be added to each word to change it into a new one:

BARGE
BINGE
MANGE
SINGE
TWINGE