1. What do the following words have in common?

STALL
ACCORD
COMPLEX

2. What do the following words have in common?

ARMY
CHINA
EARN
LUNGE
HEARTH

Answers:

1. Each word becomes a new one when ION is added to it:

STALLION / ACCORDION / COMPLEXION 

2.  Dropping the last letter of each word leaves a body part:

ARM / CHIN / EAR / LUNG / HEART