1. What do the following words have in common?

ASTUTE
DEFINITION
THIRST
LAUGHING
MONOPOLY

2. What do the following words have in common?

LEI
FEET
TALE
SEND
SIDE

HINTS:
1. Think alphabetically.
2. Two added letters, in the same position, plus a change in spelling will create new words.

Answers:

1. Each word contains three consecutive letters of the alphabet.

2. Adding DE to the front of each word and altering the spelling will yield new words:

DELAY
DEFEAT
DETAIL
DESCEND
DECIDE