Summary

Try and assemble the 3 identical pieces to make up a triangle...
This might take you (or your friends, or kids) only 30s, or you might need to think about this for a while, until you figure it out.
Please don't give the answer in the comments, that would spoil the fun ;-)

Print Settings

Printer Brand:

Prusa

                                            <p class="detail-setting printer"><strong>Printer: </strong>                     <div><p>Prusa Clone</p></div></p>                                                                                <p class="detail-setting rafts"><strong>Rafts: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting resolution"><strong>Resolution: </strong>                     <div><p>0.2mm</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>20%</p></div></p>                                                        <br>                        <p class="detail-setting notes"><strong>Notes: </strong>                    </p><div><p>There should be 3 identical pieces. Either print the provided lot, or do things your way from the single piece.</p></div>

Post-Printing

The pieces fit tightly together, so they should be lubricated to reduce the friction.
I used machine oil, but dried soap should work just as well and be better suited for kids.

Triangular Puzzle