Here’s a bedtime story for you:
Mycroft is reading Sherlock the Phone book. He get’s Sherlock to pick a name. Mycroft will tell Sherlock a story about them, based on what he can learn from their name, members of their household, and where they live. He thinks he’s just making it up, but one day, Sherlock decides to call one of the people Mycroft told him about. Mycroft was right down to every detail.
And that’s when both boys got serious about Deducing.
The end.
Mycroft is supposed to be around 14, while Sherlock around 7. How I drew their ages don’t quite match up… but that’s the idea. I imagine Mycroft is trying to get Sherlock to stop sucking his thumb, but he gives up after a while.