1. What do the following words have in common?

PLAN
STAR
SPUN
THIN
CHIC

2. What do the following words have in common?

GUM
GAZE
PLACE

Answers:

1. Each word changes into a new one with the addition of the letter K:

PLANK
STARK
SPUNK
THINK
CHICK

2. Each word changes into a new one with the addition of the letters BO:

GUMBO
GAZEBO
PLACEBO