What do you get when you cross a human finger with a paper's edge? Try: an obscene amount of pain. Why do such little cuts hurt so much?
It turns out that fingers and paper pair perfectly to produce a potent witch's brew of pain, with each ingredient bringing something special to the mix.
First, because we use them so often for tactile testing, our fingers are coated with an extremely high concentration of nocireceptors, or nerve fibers that send touch and pain signals to the brain. This makes fingers especially sensitive our "Achilles' heels" when it comes to rifling through papers.
As for paper itself, it's the perfect battleax. Sharp-edged enough to break skin, but too blunt to make a clean cut, paper carves through fingers like a dull, jagged saw. It doesn't cut deep, but this only makes matters worse: it keeps the blade riding high, at surface level, where nocireceptors that send the sharpest type of pain signals are typically concentrated.
Even worse, shallow cuts don't bleed much, so they don't readily clot and seal. Instead they remain open, exposing nerves to the air for a protracted length of time.
They just keep on stinging.