1. What do the following words have in common?

LIST
SIDE
NOTE
DAY
MAIL

2. What do the following words have in common?

PLY
AGE
PRESS
PROVE

HINT:
#2: The same two letters will change each word into a new one.

Answers:

1. The words can be preceded by A, B, C, D, and E - in that order- letters which are the first five letters of the alphabet, names of vitamins, and the names of musical notes:

A LIST
B SIDE
C NOTE
D MAJOR
E MAIL

2. Each word can be preceded by IM to form a new word:

IMPLY
IMAGE
IMPRESS
IMPROVE