Update 12/1 - v1.1 See below!
When I was a kid and bored in class, I used to pretend my pencil was a rocket launching from my desk. Now, don't get me wrong here -- I'm not trying to squelch the imagination of any little kids who dream of being astronauts some day; I'm just trying to help those pencils fly a little straighter so they actually stick into the ceiling like they're supposed to.
So what's this "convertible" business all about, you say? Well, that's the real beauty of this design: When the teacher comes walking by, you just set the dart -- er -- the pencil down on its fins and explain that it's a space-saving pencil stand. "Rocket fins? No, don't be silly Ms. Fields -- it's just a pencil tripod!"
This is NOT intended to turn a pencil into a weapon of any sort. That said, I bet this would probably make a great weapon of some sort.
The OpenSCAD file is parametric in case you need to tweak the sizes. I fiddled with it until it sliced nicely with 2-layer-thick fins (0.5mm nozzle), and that's how I saved it. I built it for the venerable Ticonderoga #2. Even though the hole is hexagonal for the pencil, the parameter specifies the in-diameter of the hexagon, so it should work just fine for round pencils, pens, or smooth rod, too.
(Yes, that's reindeer food in the background of the photo.)
This is a very quick print, and would make a great demo/give-away piece -- someone's bound to have a pencil handy at your demo!
Update 12/1/12: v1.1 You can now specify an angle for the fins so that the pencil might spin when it's thrown (untested). The new STL has this set at 5 degrees for a clockwise spin for right-handed throws.