A lot can happen in a thousand years, and sometimes a flashback is a good way to go. This is one of those times.
The night Zim was sprayed witht the mystic Poop Cola, he ran home. He had unknowingly switched his mind to Super Speed mode, and he thought he was running a normal speed. He didn't really notice that nobody noticed him, or that they seemed a bit shorter and were walking a lot faster than humanly possible as he ran through the streets in town.
As he reached his house, he stopped running, and the world seemed to be going too fast.
"Hmm..." he said to himself. "I...don't remember my house being this close to that forest." In fact, at the moment he couldn't remember much about what had happened that night. He turned the knob and hit his head on the top of the door.
"AGH! Stupid Earth door," he muttered as he ducked in. Immediatley, he heard distant squeaks, wich could only mean Gir came to greet him. He watched as his little robot servant ran in, then stopped in his tracks, gaping up at him.
"What?" Zim said, irritated.
"Master, you got TAAAALLLL!"
"What do you mean?"
Gir didn't have time to answer him. Suddenly, something burst through the door. Zim spun around in surprise, then noticed it was only Dib. Of course, he was a threat, but he wasn't really worried at the moment since he didn't have the element of surprise. That was pretty much necessary to foil one of Zim's plans.
"ZIM!" Dib yelled, clearly pissed.
"What now, Earth worm-baby?" Zim snarled.
"Woah," Dib stared at him. "What happened to you?"
"WHAT IS DIFFERENT ABOUT ZIM? ZIM IS THE SAME AS BEFORE!" He was starting to get really agitated.
"Well, first of all, you're really tall! And, also, your voice...is deeper, I think."
"YOU LIE! LIIIEEEEE!"
"And your hair," Dib ignored his usual outburst. "It's like, rooted into your head."
"Gir," Zim demanded.
"Yes Sir." Gir momentarily turned red.
"Get Zim a mirror."
"Yes Sir." Gir walked away quickly, then came back with a full-body mirror. He leaned it against the wall before returning to his usual blue. Then, he left to go somewhere down the hall. Zim stared at his new self, wondering what really had happened.
Suddenly, the memories of that night flodded back to him. Chasing the Earthling who had stolen one of his ships, going into the dark cave, being sprayed by that little beast, breaking out of the rubble that was left of the mountain.
"Gaz," he whispered.
Dib, remembering why he had come in the first place, got angry again.
"She's exactly why I'm here!"
"What are you talking about?"
"She's dissapeared, and I know you have something to do with it!"
Zim thought. An event of about a year ago popped into his mind, when people claiming to be from the future came in to stop him from ruling the universe, when it had turned out to be the Gaz-beast they needed to stop. Zim was just there to be the wife of the house or something, whatever that meant.
"Eh," he shrugged. "Maybe I do, maybe I don't. I realy don't know. Maybe it's for the better."
"WHAT THE HELL IS THAT SUPPOSED TO MEAN?"
"ZIM DOES NOT UNDERSTAND THIS QUESTION!"
"UGH!" Dib cried in exhasperation. "THIS WAS JUST A HUGE WASTE OF TIME!"
He turned and walked to the doorway, then paused and said over his shoulder.
"I'll be back for you later, alien-scum, as soon as I find my little sister."
He walked out and slammed the door behind him, and Zim shrugged. He turned back to the mirror to examine himself more, only to see Gir dancing with a pig.
"Uhh..." Zim edged around them and left down the hall, watching strangely.
He didn't know what had really happened to him, but he felt better. Stronger. Faster.
More powerful.
Note: There ya go, a flashback! Back to before Zim actually matured a bit, but don't worry, I'll be sure to slip in a few old habits every now and again in the real story.