1. The same change can be made to each of the words below to form completely different words. Can you determine what it is?

CANE
COED
COMBED
CONTENT
SISTER
VILLAS

2. The same change can be made to each of the following words, changing them into new ones.

RISK
LIGHT
ELICIT
LATTER

Answers:

1. The letters (word) IN can be inserted into each word to change it into a new one:

CANINE
COINED
COMBINED
CONTINENT
SINISTER
VILLAINS 

2. The letters F and Y can be added to each word, the F at the front and the Y at the end, to create new words:

FRISKY
FLIGHTY
FELICITY
FLATTERY

Credit: #2: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
F - Y UPON IT