The four words below share an unusual property. Can you determine what it is? The answers to numbers 3 and 4 are phonetic.

 

1. DOFF

2. ONER

3. TOFFEE

4. CON 

Answers:

 

Each word contains either OFF or ON. The OFF can be changed to ON, and the ON can be changed to  OFF - leaving valid words:

 

1. DOFF- DON

2. ONER - OFFER

3. TOFFEE - TONY

4. CON - COUGH