1. What do the following three-letter words have in common?

ADE
DON
LAY
ROT
SON

2.  What do these three rhyming words have in common?

MILL
BILL
TRILL

Answers:

1. Each word can be preceded by PAR to make a new word:

PARADE
PARDON
PARLAY
PARROT
PARSON 

2. Each word can be followed by ION to get three numbers:

MILLION
BILLION
TRILLION