 If you have a brass or a metal mouthpiece, your lips will stick to it sometimes. I've had that happen. Not fun. But if you buy a plastic mouthpiece, that doesn't transmit hot or cold, and so you put it up there and there's... Well, what about the saliva? Does that get caught up inside the horn or anything? Does it, if you're blowing like that, does it get in there? Sure, and here's the difference. For the spit that gets in your horns, on a trumpet, if you're a classical player, you have a water key on there. If you're a jazz player, it's a spit valve.