Sherlock was in his Mind Palace, John could tell. There were several telltale signs – eyes closed tightly, moving his face and hands subconsciously. All doors and windows tightly shut, to prevent all noise from getting in the way of his genius. John had been warned to read his book silently, or else.
Sometimes John envied Sherlock. A lot of the time, actually. He could just go into his Mind Palace and work out anything. It seemed sometimes like he had sorted away every fact under the sun, and he could just access them at any point he wanted to.
He wondered how Sherlock's Mind Palace looked. Sherlock had once said it had people in it, but he couldn't imagine which people, or why. He only had one friend.
John had an idea. He got up carefully and crept over to his computer, opening it as slowly and quietly as possible. He typed in a quick Google search: 'how to make a mind palace'.
Over one hundred million results. He started to pick through them, trying to find the best method.
Some time passed. Suddenly, Sherlock said from behind John, "You're going to build a Mind Palace?" He sounded amused.
John shut his laptop. "No," he said defensively. "Well, not really. Maybe a bit. Not a palace though. More like a…a Memory Bungalow."