The Call Of Nature
Saint (what else!)
It's the flexible material that does the trick. Originally, it fits in easily. Rearranged, it fits only with a lot of sqeezing. If you try to do the same puzzle with cardboard or a sheet of paper, you'll see that in the version with the gap the upper rim of the L-shaped section overlaps with the lower rim of the shape above it.
The overlapping area that gets cheated away by squeezing equals the area of the empty square.
The overlapping area that gets cheated away by squeezing equals the area of the empty square.
