G. Ghost
It's Halloween. We ought to be out solving crime or doing something to try to pay the rent on time, but instead we're at Baker Street, hosting a party. Although, I'm not entirely sure it can really be called a party since there are only a few people here – Molly, Greg, Mrs. Hudson, Sherlock and I. Mycroft makes an appearance, but he gets mysteriously called away within twenty minutes of his arrival. I'm not sure why he bothers coming to these rare social gatherings of ours; he never sticks around. And it isn't as if Sherlock would miss him if he didn't come at all – would he?
Sherlock emerges from the washroom draped in a blue sheet he snagged from the linen closet.
"Eh, what are you doing?" Mrs. Hudson questions him.
"Being a ghost!" Sherlock replies, his voice muffled from beneath the sheet.
"Ghosts aren't blue," Greg points out.
"Have you ever seen one?" challenges Sherlock.
Greg smiles. "What if I have?"
"If you had," Sherlock says, "then chances are you were so shocked by the ordeal that you would not be able to recall, with accuracy, most details of the ghost's appearance beyond what was immediately apparent and obvious. Therefore, you could not possibly know for sure that it was not blue."
Our party promptly dissolves into ghost-related bickering.