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It's called "saliva", otherwise known as spit, drool, etc.
Even when you do your best to keep your tongue away from the bit, you can't (OK, I can't) keep things perfectly dry. Also, combustion yields moisture (remeber Chem. 101?).
Keep the bowl lower than the bit and if you can't stand the gurgling noises, run a cleaner down to the end of the stem.