Call it "spit", "water vapor", "condensation" or unicorn tears...to those of us that don't use our mouth or lungs to produce sound on our instrument, it just seems gross. I used to regularly open for a regional/national touring Ska band. MANY times I watched their trombone player take the slide entirely off and tip it up in the air and into his mouth. I would usually turn away when I knew it was coming.
