I can't tell you xactly 'why' or 'how' that is exactly which chemicals are reacting to the ultraviolet energy. But... it is not just cat urine that fluoresces, all mammal urine does to greater or lesser extent, rat urine is apparently very active, and it seems human urine is more reactive than cat urine. And it is just not urine that glows, other bodily fluids such as semen,tears, and saliva do too. Blood fluoresces briefly if it is first treated or developed with chemicals made specifically for that purpose.( That's the stuff you see those so called crime scene investigators on TV spraying around like it was as cheap as cologne.) A couple of sites said that the black light didn't work so good tryin to find cat urine, especially if it wasn't fresh, even in a litter box; but when they tried it around the toilet the room practically lit up. So if your thinkin of buyin a black light for cleaning pet 'errors' from one of the sites that make up 99% of a google search combining urine and black light I think twice and do some product research. Back to the why..It seems that porphyrins and certain chemicals in proteins react to the light. OK, porphyrins, they are organic compounds that grab ahold of metals and are present in and vital to all living things. When the right one grabs iron it makes the hemoglobin that carries oxygen in our blood, and there's a whole slew of um combinin to all sorts a chemicals in our bodies, others form the green chorophyll in plants (chlophyll glows too.) Now as to how the black light makes porphyrin glow, what is goin on in what molecule or atom, I dunno. I'm in the dark, turn on the black light, I'll drink a lot of beer, you can cry and maybe by the resulting glow we can read some sites about porphyrins, hopefully by the time we get there they will have come up with an easier to spell name. I gotta warn you, we will be looking at such titles as;The 3-Dimensional Structure of Non-Planar Porphyrins; and Porphyrins, Phthalocyanines, Porphyria, Photodynamic Therapy: and reading such sentences(?) as 'A porphyrin is a heterocyclic macrocycle derived from four pyrrole-like subunits interconnected via their α carbon atoms via methine bridges.' Ow my head hurts, think I'll go draw some black light art and go to bed.