1. What do the the following words have in common?

DO
SO
TO

2. What do the following words have in common?

MOON
RUST
HIGH
CEDE

HINT:
1. The same two letters will change each word into a new one.
2. Think alphabetically

Answers:

1. Each word can be followed by UR to make a new one:

DOUR
SOUR
TOUR 

2. Each word is made up of consecutive letters of the alphabet.