The Five Trees Forest is an invisible world that is in exactly the same place as our world.
It has landmarks, like the Down the Back of the Settee Mines and baddies, wicked witches like the Big Flip Flops witch.
The only creatures that can jump between our world and the Five Trees Forest are sprites, and when they are in our world, sprites like to live in mobile phones because the batteries keep them warm, and the radio waves make them giggle.
To pick up and drop off sprites from the Forest, humans just need to hold their phone up to the secret signs that mark places in the Five Trees Forest landscape, and their sprite will hop into or out of the phone.
These were phones with NFC readers, the same technology now used, for example, in Apple Pay and Google Wallet.
Sprites talk to humans by sending messages that appear out of thin air (humans sometimes call them text messages), and ask magic questions like "If you were a ghost, who would you haunt and why?" in order to win treasure, and fight the wicked witches.
The answers to the magic questions from all the players appear on the map of the Forest, so that humans become part of the story and history of the Five Trees Forest wherever it appears.
The Five Trees Forest was found first at the Media Centre in Huddersfield, then at Norton Sixth Form College in Sheffield as part of the Off the Shelf literature festival, then at the BBC in White City and the Sheffield Children's Media Festival.