TURN FOUR INTO FIVE*
It is not difficult to get from TWO to SIX by changing one letter at a time, each time making a proper word, like this:
TWO
TOO
TOP
TIP
SIP
SIX
So TWO to SIX can be done in five steps. FOUR to FIVE takes longer. How many steps do you need to turn FOUR into FIVE? Can you do it with every letter changing at least once, including the F? How about turning ONE into TWO?