1. What do the following words have in common?

THUMB
GALLON
WALLOW
SHEER
SWAMI

2.What do the following words have in common?

WELL
TEA
SLEW
FOUR
BRED

Answers:

1. The final letter of each word can be changed to P to get a new word:

THUMP
GALLOP
WALLOP
SHEEP
SWAMP

2. Adding TH to the end of each word will phonetically give a new word:

WEALTH
TEETH
SLEUTH
FORTH
BREADTH

Credit:
The Puzzlemaster Presents Vol. 2
Will Shortz