Original characters by Jhonen Vasquez. (Didja know he came up with the ENTIRE Zim idea in one hour? I betcha didn't know that)
Story idea by Chineseisgreek2me (She asked me to write it, so I have a swelled head now. I betcha didn't know that)
Actual words owned by everybody who speaks English.
BUT! The pattern in which these words are arranged belongs to me. So, yeah. I own kinda part of this. (unless it sucks, in which case I blame it on my pet monkey, who actually types my stories)
YAY FOR ME!

Zim was in the lunch room if the midle skool, trying to disintegrate the food on his tray through pyrokinesis. It was not working well, making Zim rather aggravated. It made Dib very happy, though.
"Come on, Zim. If you're human you would eat some." Several kids had turned around to watch the spectacle. Zim's cover was in danger of being blown.
"No, Dib… I have already eaten SO MUCH of this-"
"NO YOU HAVEN'T!" Dib yelled, triumphant. "I've been taping you all day long and you haven't eaten anything!"
Zim, if it were possible, got greener. He stared at he plate of food which was leering back at him. He had only one opening to take. But it was risky. He decided to do it anyway.
Screaming bloody murder, Zim picked up his tray, swinging it around so that it hit Dib face-on. Then, still screaming, he ran out of the cafeteria, his hands raised above his head.
Dib groaned, wiping mashed potatoes off his face. Some of them stuck, turning light pink with the blood they were sucking from his face. Mashed potatoes are parasites, I betcha didn't know that.
"At least I've still got my camera," he said wistfully. But we do not care about him right now. Right now, we are following

Zim walked down the hall, being very normal as not to arouse suspicion. He glared at the Earth Monkeys around him, marveling at the fact that the entire population of Earth didn't know he was an alien. The ratio of six billion to one was starting to make him feel a bit… tiny.
Hey… wait a minute.
Zim looked around him at the other seventh graders walking around. Kids that had been shorter than him, just last week were now… TALLER THAN HIM?
He double checked to make sure his eyes were not playing tricks on him. No, they Humans really were taller.
"Why the sudden abrupt stretching? Have they been growing all along, and I have simply not NOTICED?"
Zim had to know. He ran back into the cafeteria, where Dib was still standing, near his table, trying to get the blood sucking potatoes away from his eyes. Zim snuck up beside him. Yes, Dib really was taller. Almost three inches taller than Zim.
"What? HOW CAN THIS BE!" Zim shrieked, but no one was paying attention to him.
Zim quickly got a bit of the pink potato and stuffed it into a glass container-thing he carried around to get samples of Earth-stuff. Then he ran home, his mind working furiously the whole time.

As soon as Zim got home, he jumped in the toilet and sunk down to his labs.
"COMPUTER!" he said in a whiny voice. "The humans are STRETCHING REALLY REALLY FAST! TELLZIMWHY!" "Okay, okay, don't flip out."
"ZIMDOESNOFLIPPING!"
"Right… erm… the humans are stretching as a result of something called 'puberty.'"
"More information. MOREISAY!"
"This occurs at about the time Earthlings reach the age of thirteen. They get really tall and gangly and they get pimples all over their faces and they get really clumsy as a result of their spatial awareness being all screwed up cuz they grow like an inch every day."
"An inch every single day?"
"Yes."
"More information on this puberty."
"Ummm. You don't want to know, trust me."
"Well… Can you predict the rate of growth for a certain individual?"
"I guess so, but I need some DNA."
"IGOTHAT!"
A small CD-drawer-type thing opened, and Zim dumped the bloody potatoes onto it. (that's why he took them, he anticipated this. See, he isn't ALWAYS retarded)
On a screen, a small diagram of Dib appeared next to a diagram of Zim. They were almost the same height.
"Advance one week." The picture of Dib was now obviously taller. This scared Zim a bit.
"How tall will the monkey eventually get?"
"Processing… PROCESSING!"
The image on the screen disappeared, replaced by a small 'please wait' sign. After a second, however, the screen displayed a different image. Zim still stood there, as short as he had ever been, but Dib was much taller. Almost five times Zim's height.
Zim stared in utter and complete horror. This would be his down fall. He couldn't possibly fight somebody five times his own size and win, not even with his spider legs.
Granted, Zim was pretty strong, even by Irken standards, but it would make little difference. (I mean did you see 'backseat drivers'? He THREW that kid like fifteen feet)
"I must do SOMETHING to fix this situation!" Zim screamed, and immediately sat down to try to work out how. But he didn't think of anything until about two weeks later, in the middle of Skool.

Chapter 2: Two weeks later

Ms. Bitters was spouting her usual DOOM. Most of the kids were either sleeping or wondering at all the horror in the universe and how much of it would affect them. They were all noticably taller except for Zim. Today's lecture was about the Middle ages and how they had much better disciplinary procedures then.
"And then, if the delinquent STILL wouldn't tell the truth, they would sssssssssstretch them on a rack. It was very horrible. Most of the time, bones would get broken and the person would be left paralyzed. In some records, the person was stretched to three feet taller." Zim, who hadn't been paying attention, heard this last bit and got an idea. He raised his hand.
"Yesss, ZIM?"
"If I am not allowed to use the restroom RIGHT. NOW. I feel that I may explode," Zim said in a level tone.
"That's too bad, Zim. You already used this year's bathroom breaks."
Zim looked around, trying to think of any diversion he could use to escape. Nothing presented itself, so he was forced to activate one of the diversions he had set up. Three seconds later a huge explosion rocked the skool. In the ensuing pandemonium, Zim slipped out the window and was gone.

"Zim are you sure this is a good idea?"
"Of course! It is a good idea because it was my idea and all my ideas are good!"
Zim's computer had his own ideas about that statement, but kept them to himself. It watched through mechanical eyes as Zim hooked himself up to a stretching device of his own creation.
"Ready… COMMENCE STRETCHING!"
Zim waited about two seconds, then started screaming bloody murder and thrashing around. The computer, who had been anticipating this because he thinks things through a little more than Zim, stopped stretching, and released the cuff-stretcher thingies. Zim fell forward onto his face.
"Oooowwww…." Zim said eloquently. (I love that word and I never got to use it)
Zim did not go to school the next day. He spend most of that day cursing the humans and their STUPID FUTILE machines!

When Zim finally went back to school , he was met with a very bad sight indeed. Almost everyone was twice as tall as him, having had a collective growth spurt over the weekend. Torque Smacky, for instance, was now twice as scary as he had been before. Zim was getting a very good lesson on this, because Torque and a bunch of his friends were pushing Zim around. Between the three of them they were at least six times as big as Zim.
"LET ZIM GO THIS INSTANT!" he shrieked as they lifted him up. They hung him on a doorknob by the back of his shirt, and watched as he kicked and screamed for a good ten minutes. As soon as the bell rang, they left and Zim used his spider legs to get down.
"Stupid stretching Earth monkeys…" he muttered, making his way to his first class. There, the teacher yelled at him for being late. Zim yelled right back of course.
This was basicly the outline for the rest of his day. Being short was a great burden on the Invader. And he had ther sinking feeling it was going to get worse.
He avoided Dib all day, not even drawing pictures of him during class. (Zim wasn't a good artist. In fact, when he drew Dib, he usually gave up before adding the limbs. Or head)
Whenever he started one of these drawings, Dib ended up being very very tall.
"OHTHISISNOTGOODATALLFORZIIIIIIIIIIIIIIIIIIM!"
So that day when he got home, he started working on a formula which was very difficult to make for an Irken. It was a growth formula, to make him taller than he could ever have been otherwise. But he knew before he even started that it may not work. If growth formulas were safe and easy every Irken and his mother would drink them, and the position of tallest would be utterly useless. But Zim started to make one anyway. He may not even need it.
But just in case.