I don't know for sure what it is, but it does occasionally smell like urine. Not always, but occasionally. That would lend itself to the theory that it's a mixture.
The best way to get a girl to squirt
every time is to insert two fingers into her and curl them up behind the pubic bone. Gently open and close your fingers to tap and rub the soft areas while licking and sucking her clit. It may take a bit of practice to keep your own chin out of the way. It only takes about two to five minutes of this G-spot massage combined with the clitoral manipulation to get her to gush all over your hand - and maybe your face with an earth-shaking orgasm.
Girls who have never experienced this will be very embarassed the first time it happens. The bed does get pretty wet. Just tell them that this is the way it's supposed to be and that they haven't been doing it right up to now. Then do it to her again. She'll be yours forever.