All of these stories are collected around the Internet.

Happy two-word stories:

Last Exam
First Salary
Scholarship Awarded
Free Wi-fi
Win Lottery
Deadline Extended
Mom's Cook
Job Offer
Battery Full
Cool Weather
Download Completed
Happy Birthday
Six-word stories (Dark Humour)
1. Painfully, he changed “is” to “was.”
2. Born a twin, graduate only child.
3. “Wrong number,” says a familiar voice.
4. Sorry soldier, shoes sold in pairs.
5. Buried on Monday. Died on Tuesday.
6. Failed class. Attempted suicide. Failed again.
7. Raped. Body recovered. Soul scarred forever.
8. I love him, he doesn't know
9. He hit send, then a tree.
10. Tiny hands. Tough labour. Lost Childhood.
11. The optimist wished negative blood report.

One-line stories 

1. The fool didn't know it was impossible, so he achieved.
2. "I am your son", he told the Alzheimer patient. 
3. In a life of regrets, she was his favorite.
4. He cried, "I want to be a girl, Mommy!"
5. He came back from war, some of him didn't.
6. "It's a boy, it's a boy!!!", he shouted coming out of the room,"I'm never coming back to Thailand."
7.  They took his father, only returned a flag.

8. Like every other night, the comedian came home and cried alone.
9. The axe asked the tree for its handle, and the tree complied.
10. Two men fought, we cheered. Two men kissed, we jeered.

Two-lines stories

1. Her husband pushed her after taking her to a height, despite her continuous pleading and requests.
She ended up enjoying a swing for the first time and wondered why was she ever scared of it.  :)
2. Many more massacres in anticipation as the usage of mass-murdering weaponry flourishes unquestioned. 
Thus went the headlines of the Mosquito Central. 
3. The lowly clerk was caught red handed accepting a bribe.
Miles away at home, his wife waited for her husband to bring the money for her daughter's cancer treatment.
4. Their love was different; She was happy every time he kicked her in the stomach.
Every time he kicked she loved him more. It was the love of a pregnant mother and her unborn son.
5. The sight of all the blood and mess made him extremely happy and he laughed like a maniac.

He smiled at his wife and lifted his first-born after 15 childless years.
6. She ran like crazy , praying none of them would catch up with her, as thousands of eyes were watching all around her.

She only stopped when she reached the finish-line as the winner of the marathon.

Writing about writing

"This sentence has five words. Here are five more words. Five-word sentences are fine. But several together become monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It’s like a stuck record. The ear demands some variety. Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasant rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes, when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals–sounds that say listen to this, it is important." 
(Gary Provost)