UPDATED to V3. Overall design is much thinner and easier to hide in flat places. Some other minor design improvements.

UPDATED to V2. Redesigned for a bit more durability. Also included is files for a multicolor print that can be done on a single-extruder printer.

I saw some neat rubber band powered surprise butterflies that fly out when people open cards, books, or packages. I wanted to make some that are 3d printable.

Video of the toys in action:
https://www.reddit.com/r/3Dprinting/comments/hgmtpw/i_made_some_prank_butterflies/?utm_medium=android_app&utm_source=share

Required:

Lighter
Rubber band
Super glue
To build:

Use super glue to attach the hub to the fuselage and let dry.
CAREFULLY use a lighter to soften the wing arms on the rotor and twist them slightly so it becomes like a propeller.
Install the rotor into the hub and attach the rubber band on the hooks.
You may need to clean up the hub and the rotor so that the rotor spins freely. I found that just spinning the rotor inside the hub for a while wears it in and then it works better.

The wings should be one layer or about 0.2mm thick. Different slicers seem to have trouble with single layer thick parts so it might take some tinkering to get right. In cura, i find it prints best by setting the initial layer height to 0.24mm.

To print the multicolor print:

1. slice and print "multicolor_part_1"
2. swap filament
3. slice and print "multicolor_part_2"