Horns get water in them — condensation — that must be removed. Some students seem to have more trouble than they should getting it out of the horn. It helps to have good problem solving skills.
I will always remember when I was a undergraduate, and horn numbers were low, a composition major who also played trumpet converted to horn to fill out the section. He asked me in all seriousness a question — that he could tell water was in the third valve slide, but he could never get it to come out. I showed him that he needed to dump the slide the other way.
Horn teacher Henri Kling (1842-1918) had a few thoughts on condensation that may also relate to being “smarter than the water,” found in his Horn-Schule on page 76.
Sometimes, an excessive quantity of moisture will collect in the instrument during a performance, (easily detected by a gurgling sound in the tube) interfering with the purity of the tone and the steadiness of its production. In that case, during the first convenient pause, the moisture should be discharged; but not before having first removed the mouthpiece, as the practice of allowing it to pass through the latter is a most reprehensible one and indicative of a low breeding.
I have more from Kling in my Horn Articles Online site, and be sure to take that mouthpiece out before you dump water out the leadpipe of your horn!