Ow! A paper cut!

Paper cuts are usually small and shallow injuries.

So why do they hurt so much?

A close look at the anatomy of our skin and the structure of paper reveals a few explanations.

We usually get paper cuts on our hands and fingers, where our skin is packed with neurons.

Some of these neurons, called nociceptors, detect potential harm.

These cells respond to high temperatures,

harsh chemicals

and any pressure that threatens to break the skin.

Nociceptors trigger a cascade of electrical and chemical signals

that eventually reach the brain, informing it about injury.

In turn, our brain makes us aware of injury with the experience of pain.

Also, a paper cut is not quite as clean as it looks.

A paper's edge may seem perfectly smooth,

but on a microscopic level, it's actually pretty jagged.

A thin piece of paper cuts through skin more like a saw than a knife,

ripping apart our cells.

And paper leaves behind chemical-coated particles, irritating the wound.

Another thing, since paper cuts are generally shallow,

they don't bleed or clot very much.

That means damaged tissues and neurons remain exposed.

Every time we use our hands, the wound flexes open, disturbing these neurons.

Finally, we should also consider the psychology of a paper cut.

We probably pay more attention to a paper cut because it's on a body part we use so often.

It's also surprising and kind of upsetting to be injured by something as seemingly benign as paper.

For Scientific American's Instant Egghead, I'm Ferris Jabr.