What do the following words all have in common, beside having just one syllable each?

MESS

BALL

LIST

TON

PUT

 

Answer: 

 

Each word has a single vowel for its second letter (A,E,I,O, or U) that could be replaced by any other of the five vowels and still be a reasonably common word:

 

MESS, MASS, MISS, MOSS, MUSS, etc.