Conic sections
I’m part of a mathy family — the kids were happy that I was using the 3D printer to print out a conic sections model that I just whipped together for my wife to use for teaching math.
I was happy when I discovered that I didn’t have the 5mm rod I thought I did, so I had to chuck some slightly larger aluminum rod in the lathe and turn it down to 5mm to finish the job. ☺
Update: I updated the model so that instead of holes in which to install a separate peg to hold the ellipse on the top of the cone, the peg is printed. It was silly to keep the peg separate in the first place.