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My theory on why urine, especially, is because you can get a lot of it to boil. Any "personal" body fluiids such as menstrual blood, semen, vaginal fluid, saliva, blood, etc.--well, how often can you get a mason jar full of those other fluids?--hmmm?
If you're not afraid of a little pain getting ahold of blood is pretty easy. If you need to fill a hole mason jar with your own blood thats a pretty extreme ritual. Perhaps take a little blood and dilute it with water.