Author's Note: Here be my 50 sentence contribution, most taking place some time before Korra (and abusing the comma). Some are AU, most are canon or canon compliant, and some are personal head canons (like Pema spending part of her childhood on the streets). Enjoy!
1: Air
People whispered constantly about the fact that only one of his children turned out to be an airbender, but in Avatar Aang's eyes, all three were uniquely children of the air.
2: Apples
With a wink at his sons, Aang shouted a cheerful warning before lobbing the apple pie with his bending towards the back of their unsuspecting mother's head.
3: Beginning
"She's perfect," Aang breathed, and looking down at their new daughter, Katara had to agree.
4: Bugs
Bumi delighted in the shrill scream his sister let out, and he congratulated himself even after Mom and Dad grounded him for a week.
5: Tea
The Fire Nation Princess smiled at her Great Uncle Iroh behind her cup of fragrant Jasmine, and asked him to tell her the story of her father's banishment and rise to redemption again.
6: Dark
She had never been afraid of the dark before, but living on the streets made children like Pema learn be wary of what could be hiding in the dark.
7: Despair
Forced to kneel before the rebel leader, 15 year old Kya nonetheless puts on a brave face and spits on his boots in response to his questioning though she despairs inside, knowing that her decision she made to stay behind and buy the others time could be her last.
8: Doors
Little Prince Iroh II had a habit of running through the halls, opening doors just to see what was behind them, but that changed one night after he opened his parents' bedroom door after bedtime.
9: Drink
Tenzin's eyes were glazed with sorrow and weariness after the funeral of their father, but he was a man of scruples, and he resolutely pushed away the proffered glass of wine.
10: Duty
Duty for Lin and Tenzin would eventually become a blessing and curse, both solidifying and rotting their relationship with each other.
11: Earth
Three year old Lin strutted proudly for days after her first time bending, and Toph couldn't stop bragging for weeks.
12: End
"I'm sorry Lin," Tenzin uttered softly, and Lin found (through all the pain and rage) she was actually not so surprised after all to see that their romance had grown cold.
13: Fall
Falling was never really a problem for an airbender, but Tenzin still felt his heart leap into his throat whenever one of his children (usually Ikki or Meelo) decided to randomly dive off of Oogie during long road trips.
14: Fire
Flames danced from the Fire Lord's palm as he knelt before his daughter, it's light illuminating his scar as he solemnly promised to be a better father than his own.
15: Flexible
Bumi rubbed his head and scowled up at the laughing acrobat-turned-Kyoshi Warrior, muttering "I'm really not seeing the point of becoming a contortionist to become a better fighter..."
16: Flying
"Don't worry," Tenzin smiled down at his nervous looking daughter, "This glider has held my father and I many times; it will hold the two of us just fine."
17: Food
The newly inducted Pema stared at her dish of cabbage spring rolls and rice, guiltily wishing for the umpteenth time that the Air Acolytes were allowed meat in their dishes.
18: Foot
Tarrlok knew that he was at the bottom of the proverbial totem pole in his father's eyes, but he knew that someday he would rise and become something even greater than what was expected of him.
19: Grave
When asked why he didn't establish one of the existing Air Temples as his home base, Aang couldn't bring himself to admit that thanks to Toph's earth sense teaching he could only see mass graves instead of beautiful ruins.
20: Green
Lin tried, she really did, but she could never really explain to her mother what exactly eye color was, much less the definition of 'green'.
21: Head
"You are a strange child," Lin muttered as she paused in scrubbing Meelo down in the tub, wondering if the odd lumps on the five year old's head were entirely natural or not as he proudly announced that he had peed in the bathwater.
22: Hollow
Aang wondered uneasily just how long the frail little girl ("It's Pema, sir.") had been without food and shelter; even asleep and dead weight in his arms, she felt as heavy as a hollow boned sparrowkeet.
23: Honor
"No, I was not like that!" The Fire Lord growled in response to his daughter's giggling query as they watched the actor on stage fall to his knees dramatically, chopping off the (fake) phoenix tail while crying out "Honorrrrrrrrrrrrrrr!" at the top of his lungs.
24: Hope
With a mighty sneeze born of a cold (courtesy of his cousins), baby Tenzin rattled the house and blew back his astonished parents' hair and robes, blind to the exchanged looks of stunned disbelief and dawning hope as he laughed at his new trick.
25: Light
Tenzin was not a man to forget things anyway, but he knew no matter what happened he would never forget the light in Pema's eyes the day he said "I do".
26: Lost
Pema knew what Mako and Bolin had gone through without them ever telling her a thing- she recognized the same close bond (born of protective desperation that only the streets could cultivate) that she had shared with her own sister.
27: Metal
"Metal is stubborn," Toph reminded a panting Lin, "You have to be more stubborn than the most stubborn thing on earth before you can call yourself a metalbender!"
28: New
Seeing the fresh faces of the assembled Acolytes, Aang was happy to witness the rebirth of his nation.
29: Old
There were plenty of times when Katara felt old (being 85 this year, it was a given), but she felt a curious mixture of age and ironic youthful humor whenever she heard her grandchildren call her 'Gran Gran'.
30: Peace
"Don't cry for me, Korra," Katara murmured with a sad smile, "I'm going home to see Aang- I'm not afraid to die."
31: Poison
"You have to admit, they're getting more creative," Mai drawled blandly as she examined the needle shaped dart.
32: Pretty
Stumbling over his words, a young Hiroshi Sato shyly presented an orchid to the pretty girl with jade green eyes and easy smile and asked her if she would like to join him for tea.
33: Rain
Katara smiled fondly as she saw her husband, Master of all the Elements, laughing as he allowed himself to be lead into the downpour by their ten year old daughter for an impromptu dance.
34: Regret
"I'm sorry for making you worry," Bumi said with uncharacteristic seriousness that befit the true soldier he had become and not her reckless son she once knew, "But I had to find my path for myself."
35: Roses
It wasn't often that Tarrlok found himself regretting things, but it was while finding flowers in the market (meant to bribe that stubborn donkey goat of a girl Avatar Korra) did he suddenly remember that in all his life his mother had never once been given flowers.
36: Secret
Tenzin said he wanted to tell her a secret, but when Lin leaned forward so he could whisper in her ear she was shocked to receive a peck on her cheek instead.
37: Snakes
"Get those things out of my house THIS INSTANT, BUMI!"
38: Snow
He was a man in a ten year old body, but Mako couldn't help himself when Bolin urged him to come and join the massive street kid snowball fight.
39: Solid
Uncle Aang's reassuring hand on her shoulder and the gentle reminder that it was okay to cry in front of him was the final breaking point for Lin's previously iron resolve that she'd never mourn the absence of her father.
40: Spring
It was a rare sight indeed to see the Fire Lord's wife smile at all, much less so openly and with tears of happiness as the warm spring day when her daughter was born.
41: Stable
"Don't worry Bo," Mako said, tightening his grip on his little brother's tiny cold fingers and focusing on transferring a carefully controlled warmth to him, "It'll be okay 'cause I'm here."
42: Strange
Tenzin fought the urge to reach a hand up to his now shaved, tattooed head, and instead scrutinized his reflection in wonder (until Bumi inevitably arrived with the cue ball jokes that he knew were coming).
43: Summer
Asami's favorite season was summer, for it was when the sun was high and the heat blazed up from the pavement of the race track in shimmering waves that it was the most satisfying to strap herself in and take a spin.
44: Taboo
The subject of the baby's father was strictly off limits, to the point that not even Toph's closest friends knew of his identity.
45: Ugly
The mask was there to hide the ugliness of his (false) scarred visage, but every bender who faced him was witness to a different kind of ugliness only visible in Amon's eyes.
46: War
"I know you want my help," An aging Katara responded to her son's troubling letter recounting the unrest in Republic City with a steady hand, "but the truth is my time to be a warrior has past by- I have full faith that Korra can be the warrior that I can no longer be."
47: Water
Kya stood over the bully with fists still clenched as her brothers stood behind her, reminding everyone gathered in a cold voice that just because she was training to be a healer didn't mean she couldn't fight.
48: Welcome
"I like her," Bolin admitted quietly to Mako that first night on Air Temple Island after Pema had come in to do a last minute check up before going to bed herself, "She reminds me of mom."
49: Winter
Penguin sledding would become a tradition for the Avatar's family every winter, first his children, then his grandchildren joining in on the fun until the otter penguin colony knew to (literally) head for the hills whenever they saw air nomad robes and too wide for innocence grins.
50: Wood
Aang was right, An older, wiser Korra thought to herself as she marveled at the dense regrown forest of the Senlin region, in the end, everything begins anew...