Please forgive me if I've recounted this one too recently:
My wife and I were incredibly lucky with our first - an absolutely "perfect baby." Sleeping through the night at 5 weeks. He didn't throw his first tantrum until he was 5-6 years old, and he could be caught at literally ANY time of day or night and he was nothing but giggling, smiley, friendly and inquisitive. People would stop at our table while leaving restaurants to tell us how they'd dreaded seeing us sit down near them with a baby, and at how delighted they'd been to see a polite, well-behaved little boy who nonetheless seemed very happy. No throwing of food. No spoon banging. He conducted himself, at all times, like a gentleman.
And then there was the day of exception. He was still an infant. We were driving to Northern California. As we set out in the morning he was beset by teething pain. He began crying, quickly shifting to shrieking. Absolutely NOTHING calmed him - not even for a few minutes rest. He shrieked, literally, for hours on end, and in the confines of a small, overloaded van. We stopped for ice-filled chews. We stopped for gun-numbing meds. We stopped to distract him with ocean breezes on his face. None of it had ANY effect.
By the time we neared Oakland my wife and I were at each other's throats, each convinced whatever the other did was only making it worse. He no doubt picked up on that tension, and things got worse.
The purple face, shrieking, etc. continued as we checked into a hotel and went to our room. We thought it would be impossible to continue this so long without at least growing hoarse, if not tiring himself out, or perhaps dehydrating himself. But it didn't stop.
We popped him into his crib and watched it continue. And it was at that moment that we began to imagine ourselves as a 16 or 18 year old mother. Someone alone, perhaps trapped in a 90 degree apartment instead of a nice hotel room. Someone perhaps already taking care of one or two other kids as well. And for the first time in my life, I had a glimpse of how one can reach the point at which they just run out of patience - there are no more reserves, no support system. They go over the edge and literally beat a helpless infant to death, just to get them to STOP.
I told my wife I wanted to grab that baby and throw him through the plate glass windows of the room, letting him fall the several floors to the asphalt lot below. THAT would shut him the fuck up. And she laughed. And then I laughed. And we laughed, pretty much uncontrollably, as we talked about a few of the things that would shut him the fuck up for good.
And a short time later he fell asleep. He woke up in the morning with his first teeth erupted through his gums. He returned to his normal state, and didn't leave it again until years later and that tantrum I mentioned earlier. (And that was so nakedly calculated, obviously a poor imitation of a fit he'd seen on TV or from a friend, that we simply laughed at it, and it took him just seconds to realize it wasn't going to accomplish anything.)
Hitting - whether spanking, slapping or a knuckle on the head - is the parent striking out in anger and frustration. It's not a weighed, considered ad rational punishment developed to train certain behavior, no matter how many times some folks tell themselves otherwise. "Do that again and you'll get hit" is not a lesson worth learning. It's teaching to not get hit - not that whatever was done was wrong. "Don't do that because it's wrong and creates problems X,Y and Z" is a lesson.