1. What do the following letter pairs have in common?

BO
CO
DU
LI
NU

2. Other than having three letters and starting with P, what do the following words  have in common?

PAR
PAT
PEA
PIN
PIT
PUN

Answers:

1. Each letter pair can be followed by MB to make a word:

BOMB
COMB
DUMB
LIMB
NUMB

2. Each word becomes a new one with the addition of CH:

PARCH
PATCH
PEACH
PINCH
PITCH
PUNCH