You're reading a book or opening the mail and it happens. In just a split second, you feel shooting pain and know you'll experience days of discomfort, all from the tiny edge of a piece of paper. How can a paper cut hurt so much and for so long?

You don't mean to be a baby, but dang.

There are several reasons paper cuts are so excruciating. First and foremost, blame it on nerve endings, specifically the vast number of them found in the areas where paper cuts typically occur.

Paper cuts usually happen on our finger tips, or sometimes on our lips and tongues. (Ever lick an envelope and get that sharp paper edge? Ouch!)

"The exquisite sensing abilities that makes our fingers, lips and tongue so good at what they normally do, also makes injuries all the more painful," writes Gabriel Neal, a clinical assistant professor of family medicine at Texas A&M University, in The Conversation.

These areas of the body have intricate nerve networks that are so sensitive they can distinguish between feelings of heat, cold, pressure and injury.

"Fingertips are how we explore the world, how we do small delicate tasks," Dr. Hayley Goldbach, a resident physician in dermatology at UCLA, told the BBC. "So it makes sense that we have a lot of nerve endings there. It's kind of a safety mechanism."

Reliving the pain over and over

These body parts also happen to be popular areas that we use all the time. So a cut on your finger, for example, will often reopen throughout the day as you use your phone, eat, work, drive or pretty much do anything that requires your hands.

Surprisingly, the depth of the injury is also an issue. Paper cuts are shallow, so you would think they wouldn't be so painful. But they are deep enough to expose and aggravate the nerves, but not so deep that the injury bleeds.

Bleeding causes clotting and scabbing, allowing healing to occur without a constant barrage of annoying triggers as we go through the day. But because a bleed-free paper cut is so shallow, it isn't protected and is exposed over and over again, forcing you to keep reliving the pain.

This video from Scientific American shows exactly why a paper cut hurts so much.

Paper's not so harmless

On the surface, paper doesn't seem like it should be able to inflict much pain. Although it appears to be a smooth, straight edge, that's not the case.

The fibers on the edge of a piece of paper actually give it a serrated edge, like a tiny paper saw. So when the paper slices through your finger, it's sawing through your skin, leaving a jagged path of shredded, torn damage.

Depending on the paper (a much-handled library book or well-traveled mail, for example), there's also the chance of bacteria being distributed into your skin with that see-saw motion.

That's why Neal suggests immediately cleaning the cut with soap and water and covering it with a bandage for a few days to protect it from infection and from reopening.

In your head

There's also a psychological element to the pain of a paper cut. Because you tend to see the cut or feel the dull ache all day long, it's a constant reminder that it's there.

But maybe that's a good thing, Neal writes.

"Paper cuts are trivial, but they may invoke a complex emotional response," he says. "Paper cuts remind us that no matter how many times we have performed even a simple task, we are capable of accidentally hurting ourselves. If that makes us a little more sympathetic to our neighbor's pains, and a little more humble, then maybe paper cuts do us some good too. Maybe."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Why does a tiny paper cut hurt so darn much?
Paper cuts are so tiny and shallow, but there are many reasons they are so excruciating.