In an early example of how office work can be made to sound more exciting by likening it to military adventure, one early-20th-century advertisement boasted: “Eversharp leads are smooth, strong, and fit Eversharp like ammunition fits a gun.” Yet even a premium metal-jacketed mechanical pencil, for all its military-tool bravado, has a crucial weakness. For the mechanical pencil is an exoskeletal organism. Its epidermis provides the structural rigidity within which the vital organ, the lead, is protected. And yet at the same time this thin spindle of graphite must protrude from the body to enable the user to make a mark with it, creating a point of extreme vulnerability.

If the mechanical pencil were a videogame boss, this extrusion of its intestine would be the weak spot the player should target. Here is where the lead so often breaks. This breakpoint has even been subjected to physical analysis by Henry Petroski in another of his books, Invention by Design. Consider the protruding lead, he suggests, as a Galilean cantilever beam. Assuming no flaws or nicks in the lead, it will break precisely at its junction with the pencil’s metal tip.

Many mechanical pencils have another vulnerable part. If the barrel has a clip, it is usually designated a “pocket clip.” But I for one don’t limit my pencil-clipping to pockets. I clip a mechanical pencil to the inside pages of a book I am reading, where it acts as a bookmark as well as a handy instrument of marginalia. I clip mechanical pencils to the rigid covers of Moleskine notebooks. The clips usually tear a few of the inside pages. But it is worth it, to have a notebook with a pencil always attached.

Always, that is, until the clip fails. And it always does. I have many mechanical pencils with broken-off clips. They haunt my desk, mute witnesses to my abuse. Moleskine itself makes a mechanical pencil designed to be attached to its notebooks, with an apparently sturdier clip. But this pencil has a rectangular cross-section. Perhaps it feels comfortable to the notebook, lying flat to its cover. But it is certainly not made for human hands.

What new ideas might remain to be discovered in mechanical pencil land? It was only in the 1980s that manufacturers invented what Henry Petroski calls “truly automatic pencils—ones that feed ultra-thin lead by the action of writing itself.” And to this day, the use of mechanical pencils entails regrettable waste: You have to throw away the last bit of lead when it has become too short to protrude from the tip while being gripped by the pencil’s internal mechanism. This is, as Petroski notes sternly, “one definite shortcoming of mechanical pencils that cries out for improvement.”

Yet other ingenious enhancements have appeared. You can now buy several models of mechanical pencil known as the “Kuru-Toga,” invented by Uniball. The problem its designers noticed was this: If you don’t hold your pencil exactly perpendicular to the page but at an angle, like most people do, then even the fine lead of a mechanical pencil will wear down more quickly on one side. This results in a softer, chisel-shaped point that draws a thicker line. So Uniball’s engineers dreamed up a pencil with an internal geared mechanism that rotates the lead slightly every time it is lifted off the paper. Now the lead is worn down equally on all sides and the chisel-point never appears.