When I was 4, I was with some older children playing a game in which one person thought of a city and the others had to guess what it was. When it was my turn to think of a city, I was stumped: I literally could not think of a city which had not already been used, not yet knowing many place names at all. One of the others, exasperated at my failure to produce something, exclaimed, "Just make something up!"
So I did. Already cringing inwardly, knowing how stupid it was that they should be trying to guess a name I'd just come up with, I invented the city of Twisty-Wisty.
Once they gave up on their guessing and I told them the answer, everyone lost interest in that game.
But oddly, something else started. Twisty-Wisty grew in my mind: a city of the absurd on some faraway world. Soon I knew it was the largest city and capital of Donkey-Plonkey (I have no idea why that name!), a country on Ianplan—yes, at age of 5 I felt the need for a productive suffix for the naming of planets. Ianplan as a whole was not meant to be ridiculous, but little besides Twisty-Wisty was ever developed; indeed, all that I can remember is that they had a 40-hour clock which avoided our nonsense of going through 12 hours twice a day (I drew this clock, very badly, when I was 6, I believe).
No later than the age of 8 Twisty-Wisty had an underground railway system (a bit of a failure: my intention had been to try to draw a maximally useless system, but the one snaking line was efficient at going north-south at least), and a language.