Many 3D printers can print directly from an SD card instead of being hooked up to a computer all the time. This presents a problem of transporting SD cards without tying up your hands. Or pockets. A ring that you can put an SD card in is just the thing.
While a ring you can put an SD card in is a slightly silly solution, it's an excellent example of modeling based on physical objects. Both a finger and an SD card will need to be measured, and the design will have to be modeled too:
This chapter will walk you through the process of making a cool 3D-printed project, going through the following steps:
Taking measurements
Modeling the ring
Testing the ring
Adding an SD card holder
Putting it all together
Extra credit
The challenges of matching real-life measurements to 3D-printed objects is made difficult by plastic shrinkage and inaccuracies in 3D printers' operations. I'll comment on that when it becomes important.
Some would claim that Blender isn't capable of CAD...