1. What do the following words have in common?

PAL
PAN
WIN
TAPES

2. What do the following words have in common?

SPIES
SILL
PEAS
SEPTIC

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

Answers:

1. Each word can be changed into a new one by adding the letters TRY at the end:

PALTRY
PANTRY
WINTRY
TAPESTRY

2. Inserting the letter K into each word will change it into a new one:

SPIKES
SKILL
PEAKS
SKEPTIC

Credit:
Both puzzles: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
#1: GIVE IT A TRY
#2: K TOO