1. What do the following words have in common?

LOG

COMB

TONIC

2. What do the following words have in common?

MANY

BONE

HERO

DEAL

ROOM

3. What do the following words have in common?

FILE

SALE

RILE

WALE

MILE

MULE

HINTS:
1. The same four letters, in the same order, can precede each word.

2. The same letter can replace a certain letter in each word.

3. Each word will go from four letters to six, by adding the same set of double letters.

Answers:

1. Each word can be preceded by CATA to form a new word:

CATALOG / CATACOMB / CATATONIC

2. The first letter of each word can be replaced by Z to make a new word:

ZANY
ZONE
ZERO
ZEAL
ZOOM

3. The letters DD can be inserted into the middle of each word:

FIDDLE/SADDLE/RIDDLE/WADDLE/MIDDLE