It was a small door, just next to the escalators in the Bond street station. It was quite small, with “Staff only” written on it. People were passing by this door everyday. Tube staff knew it’s for staff only, but nobody actually ever went there, and because nobody was even thinking about it, or wondering (they have lots of other thing to wonder about, much more important things: christmas shopping, holidays, household things), so nobody ever wondered what was actually behind this door.
One day, the “staff only” paper felt down and faced the passengers with another side. Some kind of instructions were written on it. The 8-years old boy was waiting for his mum finish the conversation with her friend. The boy was clearly bored. He noticed the paper and started reading it. He didn’t hear his mum calling him. She called him again. Finally he raised his head and followed his mum to the escalators. He was very excited and he was going to tell something to his mum. But she probably would not be the right person to speak about it, realised the boy in the last moment. It would be something about time travel. Adults don't believe in such nonsence.
The "Staff only" paper got lost. The tube station manager replaced it with the new one, but without any odd instructions on the back.