In June 2013 Toby Little, 5, was walking home from school contemplating the long summer ahead. Money was tight that year - his mother, Sabine, had been made redundant from her job at the University of Sheffield following the financial crash - and Toby's parents had told him they would not be able to afford a holiday.
Instead of dreaming about foreign travel, he had been reading a book, Letter to New Zealand, written by children's author Alison Hawes, where she explains what happens in an envelope's journey from England to the other side of the world.
As they neared their house, Toby asked his mum if he too could write a letter to New Zealand. After a few more steps he was struck by an even better idea. "Can I write a letter to every country in the world?"
Three years on, in their family home in Bolsterstone, near Sheffield, on the edge of the Peak District, Sabine, 40, says she often wonders what would have happened if she had replied differently that day.
"If he had asked me in the evening when I was knackered I might have refused. But as parents you realise there's a lot of power in just allowing kids to have a go. Also, it seemed a nice way to do some armchair exploration. So I told him let's see how far you get."
The answer, it transpires, was very far indeed. Toby has since written and posted letters to all 193 UN member states, as well as numerous other obscure overseas territories including Pitcairn Island, French Polynesia and the Vatican.
To date he has written 681 letters - the best of which have been selected to appear in a new book called Dear World, How Are You?, which is published by Penguin on March 24 - although so prolific is Toby's pen that that number will have risen by the time you read this. The previous week, when I met him for his first full newspaper interview since rocketing to fame, he was on 665.
"Writing these letters has made me realise that the world isn't actually all that big," he says with a grin.
It does not take long in Toby's company to be utterly disarmed by his enthusiasm. During the course of our hour-long interview, he shifts positions from an exercise bike, to peering out through his father's telescope (Nigel, 44, is a mobile phone games developer), to wielding his lightsabre, to showing off his mineral collection, to pointing out countries on his globe, to clambering across the beams in the kitchen.
He is bright, engaged and full of questions of his own. It is no surprise that so many people from all walks of life have become swept up in his project. On the living room table is an envelope that recently arrived from Bhutan, containing a letter written by two young brothers. Next to it is a piece of mail bearing a rather more regal postmark - signed by the Duke and Duchess of Cambridge and thanking Toby for inquiring about Princess Charlotte.
That first letter he wrote three years ago was not, in fact, to New Zealand, but a town called Volcano in Hawaii. Toby, who is an only child, had sat at home with his mother and picked five countries he liked the sound of. She then wrote a post on her Facebook page asking whether any friends could help them with addresses and the offers came flooding in.
"At first it was just friends," says Sabine, now back at the University of Sheffield as a lecturer in languages education. "Then it was friends of friends. And then it completely and utterly took on a life of its own. After about four months the project suddenly went viral and we had thousands of people wanting Toby to write to them. That was a bit tricky to manoeuvre because Toby was still only 5. He got 4000 messages in three days.
It was absolutely all sorts of people just genuinely touched by the innocence of it, saying he has given them back their faith in humanity."
She pauses as Toby leaps on to the sofa to whisper something in his mother's ear. "Oh yes, and we also had three offers of adoption. I think they were all from the US."
The requests are made via a contact form on the website they set up for Toby's project, called Writing to The World. Sabine vets the messages before passing them on to Toby to see whether or not he wishes to write back.
For Antarctica, he wrote to a team of scientists at the South Pole Research Station. For Syria, he wrote to Dlan, an 11-year-old blind refugee who now lives in Iraq. For South Sudan -- which has no functioning postal system - his letter was carried out between five different humanitarian workers, and the reply came back via four. For Lebanon, he was sent a mocked-up front page of its newspaper, The Daily Star, featuring Toby himself.
After completing his first mission by managing to correspond with the entire globe within a year, he decided to keep going, writing to "anybody, in any country". His first letter to Hawaii took him 45 minutes to draft. Now he rattles one off in five. A pile of half a dozen or so in his customary blue envelopes are waiting to be posted when we meet.
Each letter costs about £1 ($2.11) - a sum his parents do not take out of his £2.50 a week pocket money.
"I think people feel happy when they get a letter from me," Toby says.
"It depends on the week but probably I will write somewhere over 10. Sometimes I do a 10-letter Sunday. I've said to my mum that I might carry on and see if my child would like to carry this on after me."
Toby's missives typically follow a simple format. He inquires how the person is, what their favourite place might be, and whether they could send him a recipe.
Even three years on from the project's inception, most days a letter with an exotic postmark still lands on the doormat. His parents keep four or five plastic trunks filled with the 300-plus replies Toby has received.
Toby hopes to travel the globe in person to see some of the countries where his letters have ended up. Gambia is top of his list.
"We're so lucky to have this wonderful world," he says, looking once more at his map.
"You can explore where you want and go where you want and it's just really nice to live on planet Earth."
Who could ever have imagined that the boy from Bolsterstone would remind quite so many people of that?
The world replies
Some of the letters sent to Toby
My name is Daniel. I live in Ross River. It's great living here. How is it where you live? It must be good for you because you are having a birthday. Happy birthday!
We have wolves here. They are like big dogs. They are very mean. They also hunt in packs. We don't have the same birds as you. What do you have? The internet is fast here. I like playing games and playing outside.
I am in grade 6. I also hunt caribou with a .30-30 cal in the winter. I drive a quad. It's very fast. How is it to live in the city? This is a Kaska word [that] means see you later: nahganastanzi.
P.S. Cool Beans
We live in Setif, Algeria. Our town was named Setifis by the Romans and it has kept that name pretty much. We have five children: Aishah, 9, Abdullah, 6, Zoulaikha, 4, Khadidja, 2, and Youcef, 1. Aishah is your pen pal. I am writing for her because she speaks Arabic, French and English, but is just learning to write English. Aishah says: We have seen the fennec foxes in the zoo. They are so cute with black eyes and golden fur. They live in the Sahara Desert, about three hours from us. We eat lots of stews here over grain called couscous. Also we love cakes and cookies made with nuts, honey and powdered sugar. Kids here are just the same I think. I am on the swim team.