Transformation

A good pencil and paper word challenge. Players are given two words of the same length. It’s easier to use 3 or 4 letter words. The challenge is to transform the first word to the second in steps by changing one letter at a time, but each step must be a viable/proper word in its own right. E.g…

SOAP to BATH:
SOAP
SOAK,
SOCK,
SACK,
BACK,
BACH,
BATH.

There is always the chance that it’s not possible to convert certain words to another this way. If you find any you think can not be done why not post them here to see if somebody comes up with the solution?

If you do this a game, its the player to find the shortest route in the fastest time wins.