Scientists have discovered why when you bite your fingernails they only ever rip across, and never downwards.
Fingernails are made up of three layers
They have discovered that the central layer of the nail is reinforced by long thin cell structures.
These act to direct tears across the nail no matter which way you try to rip.
The research, by a team at Manchester University, is published in the Journal of Experimental Biology.
The researchers harvested 3mm-long nail fragments, slit them at the top, and tried tearing downwards. However, they found they could not do it.
They then used precision cutting equipment to cut nails lengthways, and across, to test their toughness.
The nails were twice as tough along their length than across their width.
They also proved to be almost at tough as horses' hooves.
Examining the nails under an electron microscope revealed that they are made up of three layers of a fibrous substance called keratin.
The top and bottom layers were both made up from "flat overlapping slate-like sheets" which could rip in almost any direction.
However, the middle layer was built up from long thin keratin-packed cells, arranged parallel to the half moon at the base of each nail.
Further tests with the cutting equipment showed this arrangement meant that the middle layer was four times tougher along its length than its width.
It is this extra toughness that stops cracks or tears penetrating to the nail's bed.
Fingernails are as strong as horses' hooves
Lead researcher Dr Roland Ennos told BBC News Online the structure of the nail was ingenious.
He said: "It makes the nail strong, so that you can use it to prise open cracks, lever things up and scratch, while preventing it from breaking into the nail bed, which would be excruciatingly painful."
He said the fact that the nail ripped across its length acted as a self-trimming mechanism, keeping the nail in a good shape.
"Even though early man probably didn't have scissors or files his nails probably looked perfectly presentable."
Details of the research are reported in the magazine New Scientist.