I have this cartoon in my head. I muse on it occasionally. It goes like this:
Panel one: Harry Potter, in a black wizard's gown with blue Ravenclaw trim, is browsing the internet and a light bulb goes off over his head. Thought balloon: "So, Dawn removes mysterious darkness! Hmmm..."
Panel two: Harry Potter is pouring a gallon of Dawn dishwashing liquid into a bucket perched on a door top. Thought balloon "I'll only get one chance to make this work."
Panel three: Harry calls through door. Word balloon: "Professor Quirrell, could you come in here, please?"
Panel four: Quirrell is drenched in blue liquid as he comes through the door. He scowls.
Panel five. Quirrell beams. Word balloon: "Harry, I finally understand! We can all just be friends!"
This little sequence has given me a lot of pleasure. Of course, it is a seriously deep in-joke. It tickles me because I have read two items, both of which are known to small groups, and I suspect those groups barely overlap. The first necessary piece of background is Harry Potter and the Methods of Rationality, a fan fiction in which Harry has loving parents who introduced him to the science of decision making before he ever received the letter to Hogwarts. In this excellent story, Harry joins Ravenclaw rather than Gryffindor, and becomes friends with Professor Quirrell, who suffers from mysterious darkness. The second necessary piece of information came from the discussion of using a tumbler to brighten chain maille on the Urban Maille website, where Aislyn Bryan asserts that the cure for mysterious darkness is more Dawn.
If you've ever wondered where science fiction authors get their ideas, there's one example in thorough detail for you.
Part of the pleasure of in-jokes is the recognition of shared background when someone else laughs at them. "Eureka means this water is too hot" is lovely for that. To laugh together means Yes, I see you; Yes, I hear you; Yes, we both know this well enough to recognize and enjoy referring to it. It can be a wonderful moment of connection.
Doug understood the comic from its description without explanation. Sharing a lot of inside information is one of the benefits of long-term relationships.
And I would be glad to invite you into the circle. Both the websites above are excellent on their topics. I strongly recommend them for outstanding storytelling and quality chain maille information, respectively.