1. What do the following words have in common?

AT
ON
OR
FLU

2. What do the following words have in common?

AID
AIR
EAT
LAY

Answers:

1. An E can be added to each word to form a new one:

ATE
ONE
ORE
FLUE

2. Each clue can be preceded by REP to form a new word:

REPAID
REPAIR
REPEAT
REPLAY