system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does sperm show up under a blacklight?
gpt
Technically it's not the sperm that is glowing.  It's the semen.  All organic material, especially organic fluids glow under blacklight.  Phosphors are materials that emit visible light under UV radiation.

Organic (based on carbon) material almost always has phosphors present.  There are many phosphors including oxides, phosphorous, nitrides, sulfides, and countless others that respond to ultraviolet light.  That's what blacklights are.  UV-A emitters.
All of the above elements and compounds are present in organic material such as your body.  Semen is no exception.