The retractable pen is an interesting little device. You press a button, and the tip comes out. Press it again, and it does the opposite: the tip disappears back inside. How can the same action lead to different results? In this post, I'm including a CGI animation I created using Blender. I actually modeled this pen in early 2012 (a little over 4 years ago); I recently fixed it up and re-rendered it. A retractable pen has 5 main parts: 1. Frame 2. Thruster 3. Ink cartridge 4. Spring 5. Cam (a special mechanical piece; 1 or more is used) The basic design is for the cam to rotate each time the pen clicker is pressed; the rotation allows for a pin (built into the frame) to slide into a different slot. Different slots have different lengths, and depending on the length of the slot, the ink cartridge will extend by a different amount. There are many ways to work out the details, and the animation I created shows only one possibility. Of course, the design of the pen in the animation wou