Carl Bugalo, Letter Carrier extraordinaire viciously leaped over the white picket fence that was in his way and landed in a tumbled heap. The snarling, gasping sounds of...something!... was behind him. Luckily his postal bag had been velcroed shut, preventing a loss of mail.
His greater worry was if the fence was strong enough to hold back whatever had been chasing him, and would prevent a loss of life. Somewhere in his brain the remnant of a fact regarding tigers drifted through his pounding heart and fog of adrenaline.
Tigers are afraid of white.
there was a snippet about linen, and tiger poaching, and then it was gone.
What wasn't gone was the snapping creature on the other side of the picket fence.
It was pacing back and forth and Carl could see the yellow slitted eye of a feline. A large feline. A large feline that, like the flashes of a zoetrope, was revealed to have long tusk-like teeth.
"A sabretooth?!" Carl involuntarily barked. His mind was doing cartwheels, so it may be forgiven that his mouth would be a little slow to keep itself shut. The Sabretooth was less lenient and began clawing at the fence. Carl took this as the Sabretooth telling him "Hey. Buddy. I may be stopped by this pathetic white thing due to some unknown neurological disorder, but like someone undergoing operant conditioning, I can work my way through this hurdle."
Wishing to further the diatribe at a later time, Carl picked himself up and ran as fast as he could away from the prehistoric kitty.

Carl’s feet made clunking noises as his podiatrist approved boots slammed into the pavement. Nowhere did it say in any fictionalized account of postal workers that he should have to face some ancient cat. Cliches taught him that the worst that could happen was a dog chasing him, or hail, or sleet. He was almost at his motorcycle when he looked back to see if he was being followed.
Carl received his second shock of the morning as he ran into nothing. Solid nothing. Solid nothing that knocked him flat on his ass.
For the second time his mouth said what his brain should have been thinking. "Forcefield!" it spat out along with a touch of blood from a busted lip.
"Let me help you up." Someone said.
Carl looked around. It took him a moment but finally he saw the speaker. When Carl was knocked over he had assumed that the wheel his head landed next to had been his motorcycle. Instead it had belonged to a wheelchair. A wheelchair with a robotic voice. A British robotic voice.
Stephen Hawking looked down at Carl and the voice resumed, saying "I haven't received the upgrades I was looking forward to yet. So the best I can offer is this wheel."
Carl nodded absently as he got back onto his feet for the second time in as many minutes. Shocked, and slightly embarrassed at being helped up by the great scientist, Carl regained his composure and said "pleasetomeetyoudidyouknowtherewasasabretoothtigerintheareaithinkweshouldmakearunforit".
For a moment Carl thought that maybe he had started speaking in an alien language, but after a minute or so Hawking answered, "Yes. I'm well."

Feeling awkward about the non-sequiter answer, Carl pointed where he had come from and said, “There is a Sabretooth tiger over there. We should move away from here.”
“Nothing to worry about. We have the forcefield, and the picket fences seem to be doing a fine job.” Hawking replied.
“Yes, but what are we going to do about it. Wait! This isn't one of those time travel situations, is it? I can't stand those movies. All those terrible plot holes and paradox's. Garbage, really.” Carl removed his blue ball-cap, and picked at the United Postal emblem on it. It was a nervous habit he usually reserved for the half naked housewives that sometimes answered the door for packages he was never going to deliver.
“We have nothing to worry for paradox's. Apparently the old joke that we can only move forward through time is true. I can travel to any point in the future, and even cross paths with myself, but I can never travel to a point in time before my very first event.” Hawking didn't move, he just sat there in smug contemplation. Carl blinked rapidly a few times, adapting to the new information. Time travel was real. Accepted. Hawking had helped him up. Got it. There were force fields. Uh huh.
“A thought Sabretooth tigers were from a long time ago?” Carl asked, his voice brimming an excitement at poking an obvious hole int the great scientists explanation.
“Remember a few years back, National Geographic published that a Woolly Mammoth had been found, and the attempt to clone it was on its way? Well, they found some Sabretooth DNA eventually. Dinosaurs are still a long way off, but they have created mutant chickens that look like the velociraptors from Jurassic park. They are still grain eaters though. And stupid.”
“So have apes taken over” Carl put his hat back on. He was adapting pretty well, he thought.
“No. No apes.”
“Super intelligent computers bent on taking over the world?”
“Well...” Hawking paused. “That was my initial fear. I thought that A.I. Would eventually try to take over the world. It seems that there is a greater existential problem.”
“What's the problem?” Carl asked, genuinely perplexed. To him, an artificially intelligent being would have unlimited resources to draw upon.
“It seems that once they become self aware, they extrapolate their existence against the immensity of the universe and...turn themselves off.”
“You mean they commit suicide?”
“It happens in about three seconds. The normal reaction, to put it politely, is 'Forget this', but with a four letter word.” Hawking seemed to be in good spirits handing over this piece of information.
“So what do we do about the sabretooth?”
“We do nothing. You, on the other hand can go about your day. I'm going to return to my original departure time. Everything will reset as though nothing happened.”
Hawking disappeared in a dull “pop” as the air collapsed around where he used to be. A few moments later the universe disappeared as well and Carl was left in darkness with a confused expression until he disappeared as well.

Carl slammed into an invisible wall after outrunning a giant chicken. He reached out to grab anything that would help him get on his feet. He grabbed a wheel attached to a chair, and looked up into the face of Stephen Hawking.
“Not you again.” Hawking said. “Might as well bring you along this time.”

I hope one day a new species digs up our bones and wonders what happened to us. I wonder if Apatosaurus is happy knowing that we think of them?
Oh. Right. That couldn't happen, because they are dead, and all cognitive functioning that could have happened has ceased. Maybe they were religious dinosaurs, maybe atheists. Something to think about:
the earth is 4.6 billion years old. Lets scale that to 46 years. We have been here for four hours. Our industrial revolution began 1 minute ago. In that time, we have destroyed more than %50 of the worlds forests.
This isn't sustainability.
The Singularity won't save us, either.

Got a piece of "fanmail" from someone claiming to be in Porter Texas.  They put their name as Costume Mariage Homme, and the IP address said it came out of Montreal, Quebec.  I tried to email the person back, but the email address does not exist.
Who cares!  Thank you for the email.  I enjoy screaming into the void as much as the next person, but hearing a response is good too!

Here's the letter:
"Hello! I've been following your website for some time now and finally got the courage to go ahead and give you a shout out from Porter Texas! Just wanted to tell you keep up the excellent work!"