About that slapping-Clark-without-getting-hurt -thing, I've always thought that it depends on *how* you hit. I mean, you can easily slap/hit with your palm at something made of steel (let's say a car, just for the sake of it) without getting hurt. I've done it a couple of times with all my might, when something has annoyed me. And sure, it stings *a lot*, but I don't get hurt. In the other hand, if I'd tried to punch the living daylights out of the car, then I'd probably broken my hand.
So, my bet is that it probably depends on how you hit your mark. But, Lois' hand really should have ached quite a lot after that slap...but in the other hand, she's mad and probably running on adrenaline. Both quite good at hiding pain.
