Having something "click" in a way that makes the chain of resulting understandings overwhelming. EG: "Wait... you mean c is c because there's a relationship between time and space, so that the speed of light is really a product of the ratio between time and space?"

Click.

"So THAT's what c is doing in E=mc^2, and why time slows to zero and mass increases to infinite and distance contracts to zero as you approach c. And you can't go faster than c because, from your point of view, you will have reached your destination before you had time to accelerate any faster, AND the slowing of time is the reason why a man on a train traveling at near c who turns on a flashlight will perceive the light to travel at the same speed as someone standing on the platform..."

and... and... and... until I'm pacing back and forth, explaining it back to myself, snapping my fingers at each new rock this new crowbar can turn over.

Hours later I'll realize I've been in a fugue for most of the day, there's twelve tabs open in the browser, many of them Wikipedia pages, and I've written 2,000 words of essay without effort.

It happens for culture, too, particularly when I recognize an actor, look up what else they've been in, and start laughing like a madman.

"AHH!! Billy Mumy played Lennier in Babylon 5, but he was the original Will Robinson from Lost In Space. AND he was the omnipotent kid in The Twilight Zone. AND he was the guy who sung the 'Fish Heads' song!"

I've started to call it "Actor Nerding". It's like your head explodes.