1. What do the following words have in common?

HAY
RILE
PEER
ARISE
SKIING

2. What do the following words have in common?

FUR
RUB
EARL
IRON
READ
WEAR

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

Answers:

1. The letters PP can be inserted into each word to form a new one:

HAPPY
RIPPLE
PEPPER
APPRISE
SKIPPING

2. Each word can becomes a new one when a Y is added at the end:

FURY
RUBY
EARLY
IRONY
READY
WEARY

Credit: Both puzzles: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
#1: P's OFFERING
#2: Y's UP