Though Yuriko who had been five years old at the time wouldn't know it, she had changed the world with a simple decision. Whether or not the change she caused was for the better or for the worse remains to be seen, but she certainly made it more interesting for some of its inhabitants though. How she had changed the world was that during a game of tag, she had turned Left instead of Right.
How could something like this change the world you ask?
Well, it was sort of one of those perfect storm events where the timing had been just right. She had made her Left turn the day before the Uchiha Massacre. Had she turned Left a day earlier, things wouldn't have transpired the way they had, and had she made the turn during the next game of tag that she'd played which was two days later, it wouldn't have mattered because the person she had changed that day would have been in the hospital. Had she simply made a right turn on that day, none of the events that would follow would have occurred either.
How events played out was thus:
Yuriko had run left rather than right as she normally would've done, and instead of running through a maze of alleyways until she got caught, she ran right into Uchiha Fugaku while his younger son Sasuke was bringing him the lunch he'd forgotten at home that morning. Instead of backing off and apologizing as any sane person would do after slamming into the head of the Uchiha Clan and nearly knocking him over, she used the the Chief of the KMPF as a climbing structure and perched herself on his shoulders.
Naturally, Fugaku was pissed. Too pissed to speak in fact. It was only the fact that they had been in public that had saved the girl from the hiding of her life since one didn't hit small children that didn't belong to you where there were people watching, especially not small civilian girls. As it was, he had gripped the child's wrist too tightly, and nearly broke one of the bones in the girl's hand as he pulled her off of him and yanked her over to the nearest officer and ordered him to get her out of his sight.
The officer, having been amused by the girl's antics, led the child off without any further punishment.
What Sasuke saw was this:
A girl came out of nowhere and started climbing up his dad until she was perched on his shoulders from where she stuck her tongue out at the boy who had been chasing her. Rather than yank the girl off and throw her to the ground, give her a spanking, and yell at her about how she wasn't to do that ever again like he had done to him when he was three after he'd climbed on top of his father's head and got mud and gum in his hair, his father had gently pulled the girl off of him and led her over to an officer who had led her away with a smile.
After thinking about this for a while, and about what he'd seen of his father's interactions with women and other girls amongst other things, he came to a conclusion. His father hadn't been disappointed with him because he hadn't been as good as Itachi, his father had been disappointed with him because he wasn't a girl.
How Sasuke had added two and two together and got banana had been because his train of thought had ran thus:
His father already had one son, and therefore he hadn't needed another. It was clear from the way the man treated them, that he liked girls better than he did boys. Apparently, he was supposed to have been a girl, and had failed his father by not being one.
With his failure in the gender department in mind, and his current lack of knowledge when it came to exactly what it was that made the sexes different keeping him from realizing that what he was thinking of doing was impossible and likely would not be looked upon favorably, he came up with a plan to make his father happy by finding a way to become the girl his father had wanted. The first step in his plan had been to go home and ask his mother about girls. Since his mother had been one, he figured she would know everything about them, including how you became one.
"Why do you want to know so much about girls?" his mother asked after fielding a million questions on them from why they dressed the way they did to what they ate.
"Do you want a girl?" Sasuke asked, deciding that he didn't want to make his mother unhappy by deciding to become a girl to make his father happy, since he liked her more because she liked him more than his father did. His mother had told him many times that she loved him just the way he was, but if his mother wanted a girl too though...
"I did." Mikoto said sadly, coming to the erroneous conclusion that her son was asking for a baby sister, and that she would have to explain why that wasn't going to happen.
"What happened?" Sasuke asked, feeling sad when he saw what he took to be disappointment on his mother's face, and coming to believe that it was because he hadn't turned out to be a girl, and that the stork he'd heard Mariko's mom talking about had refused to make any more deliveries like it had at Mariko's house.
"After Itachi, I had a hard time having you. After I had you, I learned that I couldn't have anymore babies." His mother said sadly.
When she noticed that Sasuke looked like he was blaming himself for the fact that he couldn't have a little sister, and was starting to cry, Mikoto decided to cheer him up with a funny story. This turned out to be a bit of a mistake however.
"You know." his mother said with a bittersweet smile as she recalled times long past with a friend she had been growing distant from before the friend had died, because their lives had been carrying them in two different directions. "When I was expecting you, I thought you were going to be a girl, and one day, friend of mine and I went out and bought up every little pink item we could find. When you were born, we had to use Itachi's hand-me-downs and give everything to a cousin of mine who was expecting around then."
"You know, if you had been a girl, you would have been named after my friend." his mother continued sadly, remembering Kushina's surprise over the fact that she had Sasuke, and that Sasuke was a boy rather than the girl they thought he'd be. Despite the fact that she hadn't been known for it, Kushina had rather tactfully avoided the fact that she'd thought she'd had yet another miscarriage when she hadn't heard the announcement about Sasuke's birth since Fugaku hadn't made as big a deal out of it as he had Itachi's. That last time she'd seen Kushina had been the first time in several months since her duties towards her clan had taken up much of her time, as had Kushina's work as the Hokage's unpaid cook, housekeeper, secretary, and Advisor.
"What was your friend's name?" Sasuke asked, pulling his mother out of her musings.
"Kushina." his mother replied sadly. "Her name was Kushina."
Had these events and this conversation taken place a week or even a day earlier, nothing would have really changed in Sasuke's life, and therefore the world would not have changed in the slightest. Sasuke would have been caught dressing in clothes that belonged to one of his female cousins, and his parents would have "corrected" his behavior. As it was however, virtually all of the Uchiha Clan was killed before Sasuke could be caught dressing in gender inappropriate attire and have a discussion with his family that would end with him swearing that he'd never do something like that again, and Sasuke ended up being left with a burning desire to avenge his clan, and the gnawing guilt over the fact that his parents had wanted a girl and had gotten him instead.