1. What do the following words have in common, other than starting with T?

TIN
TEN
TORN
TREAT
TOUGH

2. What do the following words have in common?

POT
LOT
RAT
LEG

3. What do the following words have in common?

TAG
YEAR
TIRED
HOUSE
POUND

Answers:

1. Each word can be changed into a new one by inserting N as the second letter:

THIN/THEN/THORN/THREAT/THOUGH

2. Each word can be changed into a new one by adding ION:

POTION/LOTION/RATION/LEGION

3. Each word can be preceded by DOG to make a new word or phrase.