Mystery: Type a physical noun.

Answer: light

Crack: This riddle is guessed by knowing.

The mystery of "leaving no decency" cannot be regarded as "leaving no decency" according to ordinary thinking, but needs to be interpreted as:

Don't stay, be decent. As we know, from the perspective of three-dimensional space, it is a point, line and surface body. From point to line, from line to surface, and then from surface to body.

Then, now we leave nothing decent, nothing decent in the four elements of point, line and surface. What else is there? Ha, the light (only) left a line! -answer: light.