system
Given the following post, respond as if you're commenting on Reddit:
human
How does a blacklight work?
gpt
Black-lights shine UV light which you cannot see. There are many molecules and complexes that fluoresce under UV light.

What is happening is the UV photons are absorbed by these molecules which very briefly go into an excited state. They then lose some of the energy by non-radiative processes before emitting a photon that is slightly less energetic. If the difference is great enough the emitted light will be in the visible spectrum.

look up fluorescence on Wikipedia for more details.

If you mean "How does a black light produce UV light" that is down to the nature of the phospor material used in a fluorescent bulb.