What do the following words have in common?

1. STRING
2. ROSTER
3. CARTON
4. CURTLY
5. LASSES
6. CORNET

Answer:

Each word can be changed to a new one by inserting the letter "O" somewhere in the word:

1. STORING
2. ROOSTER
3. CARTOON
4. COURTLY
5. LASSOES
6. CORONET

Credit:
Will Shortz
The Puzzlemaster Presents Vol. 2
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