Mary;

You'll have to forgive, but this story was told to me by my Nana many, many moons ago. She's a lovely little Englishwoman, and I doubt I'll do this justice...

Why Cats Always Wash After They Eat And Not Before
Way back in the way back, when cats were cats and mice were mice, a hungry little kitten thought that a teensy little mouse might make a nice snack. Cat crept up and pounced, catching TeensyMouse in his paws. TeensyMouse cried, "What are you going to do to me Cat?" And the Cat replied, "Why, sir, I'm going to eat you!"
Whether it be brains or blind luck, TeensyMouse got an idea...."You surely aren't going to eat me before you wash up are you?" he said. To which the cat pondered a minute, cat ettiquete being what it is, and lifted his paw for a lick. TeensyMouse took that very opportunity to dart away.
Cat was right pissed, and made note to tell all of his cat friends about the incident. The CatCouncil voted at their next meeting that all cats must furthermore wash AFTER meals and not before....