Last night, I was watching a Superman cartoon with my daughter and it occurred to me that anybody who ever shook hands with Clark Kent, or patted his cheek, or playfully punched him in the arm should have known immediately that something was different about this guy.
Bullets richochet off him. Knives snap against his chest. He shrugs off artillery. Touching him should be like touching a statue or a suit of armor. As the mythos says, at least sometimes, Krypton had higher gravity and a red sun. Kal’s tissues are, it seems, greatly denser and harder than that of an Earth human.
I know, he never gets caught out on this because the writers didn’t want to do it that way.
Apparently, he’s not denser than the average human. And his body is soft and squishy. However, thanks to his magic alien powers, he can’t be hurt by any normal attack. Bullets can’t penetrate his skin, so they ricochet after failing to expend their kinetic energy pushing him back (and only getting slight give from his skin and muscle). Knives likewise will break if you push them too hard. Aertillery can actually push him around a bit, though it won’t hurt him.
Plus, even as Clark Kent, the guy is beef.
He’s frequently described in the comic as being hard as steel. Even the 1950s TV series has him described that way, at one point.
Of course, it probably depends on whether he’s tensing his muscles, or something. I can alter how hard my muscles seem, and I’m sure he can, too. Otherwise, LL would fee;l as if she’s embracing a statue.
As I recall, however, Clark actually does go out of his way to feign weakness. He will often purposely affect a weak handshake and when he detects someone about to playfully give him a punch, he will flinch away so that the other person doesn’t land a solid blow. At least it was that way early on Superman’s run where the writers portrayed Clark as more of a wimp than they might nowadays.
In one comic from the late 1950s/early 1960s, Superman actually made a fake qarm out of rubber and lay buried in the sand (as Clark Kent – he had his glasses on) with the rubber arm out so Lois could feel it and see how soft it was, the implication being that Superman’s arm didn’t feel as soft.
From: Superman (Clark Kent) | DC Database | Fandom
I would say that he’s as pliable as a normal human but once the force goes up to the level of damage his defenses kick in. It doesn’t make sense, but neither does laser eyes or his method of reaction-less flight.
He’s made of non-Newtonian putty. Harder you smack it, the more it resists. Touch it gently and it’s pliable.
He is if your name is Lois Lane.
Now that’s just Silly. 
Of Course he’s non-newtonian. Superman is made of Kryptonian fluid.
It makes perfect sense. All materials exhibit variations in the response as you ramp up the force applied. In Superman’s case, his skin could possess a normal region of elastic deformation, beyond which futher force does next to nothing.
Only parts of him, and it depends on what parts you touch.
Of course he is – after all, he is the Man of Steel.
Lois said he was hard at times yet still gentle and she should know.
This reminds of the wonderful conceit on All Star Superman of Lex Luthor going on and on about what a Promethean genius he is, and being completely oblivious to Clark Kent and Superman being more of less identical, all the while lecturing Kent on what a schlub he is.
This is of course a main meme for Superman in general, but the juxtaposition was well done.
He’s the Man of Cornstarch!
That sounds vaguely digusting.
Personally, I prefer the idea that he is, in fact, extremely dense (comparable to the density of a white dwarf), but that he’s continually using his flight power subconsciously to support almost all of his weight and keep from falling through floors.
Then why doesn’t he fall through floors (or expand in a most dramatic fashion) when he gets de-powered?
On the whole, I think the “defensive aura” thing is probably the explanation least contradicted by canon.
It’s magic of course. But since it’s canon in the comics that he can lose his powers–even the invulnerablity–for psychosomatic reasons, I suspect that when he’s dressed as Clark and not desiring to be Superman, he doesn’t feel any different than a normal human to the touch, but if he feels threatened the invulnerability snaps back to normal.
Okay, I guess he needn’t be dressed as Clark. Otherwise Lois would be really frustrated.