The Almost Impossible puzzle applied to a heart.
Print the 4 parts and assemble (if you can!).
The openscad has a parameter that defines the spacing between the pieces. The supplied STLs were generated with a spacing of 0.25mm. Depending on your calibration (and whether you are using the buggy Slic3r version) you may need to tweak this to get the pieces to go together well. I printed using the git version of slicer that implements an equivalent of skeinforge's comb function, and the pieces fit quite snugly.