To Be or Not To Be - like the Cat?

This morning, my son who is in the age of discovery noticed that the cat was, in fact, licking himself.  Now, Nathan has no idea why the cat licks himself, just that it looks like a good time.  So, using his ever-exploring brain, he decided to lick the cat as well.  When I told him not to lick the cat (yes, I had to tell my child not to lick the cat), he promptly proceeded to lick me.  Not understanding when I told him not to lick me, that was gross, he went through the toy box, licking every interesting toy he felt like.

So, at such a young age, how do you explain to a child not to lick things, but the cat licks himself to stay clean.  However, I did not tell him that was WHY the cat licks himself, because I do not want Nathan licking himself in the bathtub...