Ballon Tie Knot
FleetingI first tried https://www.thingiverse.com/thing:4921077 in PLA 100% infill. It broke easily and still needed some complicated move that my kids did not manage to do.
Then I tried https://www.thingiverse.com/thing:4899580 and https://www.thingiverse.com/thing:3118389
They kinda work, but they still need some strength and have a step that is difficult to achieve.