We've all used clicky ballpoint pens. I'll admit, I fiddle with one on a daily basis, clicking it over and over—those clicks are very satisfying. But what are the details of the mechanism that allows the tip to be extended or retracted? Although I've taken apart many of my pens (another nervous habit that I enjoy, especially during long meetings), I never spent much time thinking about the engineering involved.

It turns out that there are various different ways to build a retractable pen. But in this four-minute video, we see how the hugely popular Parker Jotter ink pen works, using a series of surprisingly clever cams and a spring. Let Bill Hammack, The Engineer Guy, explain:

Note: If you'd like to read the original patent, Google has it (complete with plenty of minor scanning errors in the text). The illustrations are beautiful.