Superman #100

I believe this is the first issue to celebrate a "hundred" milestone; Batman #100 followed a similar pattern several months later. As the Silver Age wore on these "anniversary" issues became more common, with Flash #200 sprinkling "200" signs everywhere, and Fantastic Four #100 featuring the return of just about every villain the FF had faced in their prior 99 issues.

Update: Well, the first DC issue that I am aware of; as pointed out in the comments by Bob, the 100th issue of Whiz Comics by Fawcett was heralded on the cover:

Although actually even though this was Whiz #100, it was actually only the 99th issue, as Whiz started with #2.

Captain Marvel Adventures and Captain Marvel, Jr., also featured 100th issue covers.

The opening story is The Toy Superman Contest, featuring small Superman dolls that can perform stunts just like the Man of Steel. Since a portion of the proceeds are going to charity, and as the dolls appear to be harmless, Superman agrees to judge a contest where the winner is supposed to come up with the cleverest feat that Superman can perform.

Supposing a car is stuck in a swamp, and there's a meteor of Kryptonite nearby. How can Superman rescue the vehicle? One kid gets quite elaborate:

But when it comes time to award the prize, Superman makes a very odd choice:

There are rumors that the fix was in, and the kids drop the Superman toy figure in protest. Why did Superman make such a bizarre choice?

Well, the explanation is a little complicated. There was this crook who was hiding out in the waterfront district, but he had wired booby traps in innocent homes nearby that the heat of Superman's X-ray vision would explode, preventing Supes from looking for him that way. Harry had placed a bit of asbestos nearby to prevent the Superman robot from causing an explosion in the chemical factory. Similarly, Superman uses asbestos-treated eyeglasses to prevent the heat of his X-ray vision from exploding the bombs the crook has placed, and is able to capture him.

Comments: A bit on the silly side; how exactly do the Superman dolls fly, or do any of the other feats they are shown doing? And the explanation is a little fishy, especially since it involves Superman being seen with eyeglasses on, which we were assured in other stories would surely reveal his secret identity:

The second story is Superman, Substitute Teacher. Clark is walking down the street one day, when he sees workers moving a piano from an upper floor of a building. Noticing the wire about to snap, he springs into action:

Note that this was before Weisinger and his writers introduced the idea that Superman stored his Clark Kent clothes in a pouch of his cape. But when he's changing back, a man spots him. Supes changes his appearance quickly, giving himself a mustache and snapping the earpieces off his glasses. But according to a young man passing by, this makes him look like Mr Cranston, a former substitute teacher. A principal happens to hear this and is desperately seeking a sub. Supes would ordinarily decline, but Lois is also passing by (apparently this is all happening in the Grand Central Station of Metropolis), so....

As you can probably guess, the class does indeed turn out to be mischievous, but Clark, freed from worries about exposing his real identity has some fun with the kids. He eats a wooden apple that one of them brought to trick him, and when the lock to the textbook closet doesn't open:

When one of the ringleaders tries to incorrectly answer a math problem at the blackboard, Superman uses his super-breath to force his hand to come up with the correct solution. This convinces the rest of the class to do their work accurately, and so he has apparently tamed the group. In the end, Superman exposes himself, explaining that he wasn't really Mr Cranston, and if he was a real teacher, he could do much better:

Comments: An entertaining story with some humor, possibly inspired by The Blackboard Jungle, which came out earlier that year, although of course the kids in this story are Boy Scouts compared to Vic Morrow and his gang.

The last story is The Clue from Krypton. Clark is promised a scoop if he visits a man. When he does, the man says that he knows Clark is actually Superman. He gave himself away in an autobiography he wrote. The story then goes into a flashback sequence showing how Jor-El warned of the destruction of Krypton and sent his son to Earth. Of course, this is a relatively painless retelling of Superman's origin story.

The man reveals that he discovered a small fragment of Superman's rocket that contained the baby's fingerprints. He compared them to some boys that grew up in Smallville and later moved to Metropolis, which was how he identified Kent. He wants an oil well and the world's largest diamond to keep the secret.

But when Superman gives him these items, the crook still refuses to hand over the proof of his secret identity. So Superman drains away the oil and shatters the diamond by singing a particularly high note. Then, when the man calls the press to announce Superman's secret identity, he uses the heat of his X-ray vision to change the prints just enough that they are not similar to Clark's:

Comments: This story is a different way of retelling the Superman origin. I found it amusing that the crook would have gotten away with his crime had he not reneged on his promise to give Superman back the fragment of the rocket with his prints on it.