What do the following groups have in common?

1)
FLIP
COLD
DRUG

2)
DEEP
CATCH
SWEET
STRIKE
FORTUNE
FAHRENHEIT

3)
POLL
GUST
MUCH
DINER

Answers:

1. Each word can be follwed by a word that means a failure:

FLIP FLOP
COLD TURKEY
DRUG BUST

2. Each word is commonly followed by a number:

DEEP 6
CATCH 22
SWEET 16
STRIKE 3
FORTUNE 500
FAHRENHEIT 451 

3. If you add an O at the end of each word, it becomes a word in Spanish that has been integrated into  everyday spoken English - at least in certain areas.

POLLO  (chicken)
MUCHO (a lot)
GUSTO  (mucho gusto - my pleasure)
DINERO  (money)