When they tap the metal part with their beaks a drop of water comes out.
It stays clean under nearly any circumstances, is highly leak-resistant, and is compatible with deicers in the winter for people who need that.
I live in a warm climate, but I'd had my waterers partially-freeze and though the horizontal nipples stopped functioning they didn't break.
Vertical nipples are also available. They are easier for chicks and smaller birds to use but more likely to leak and more subject to damage from freezing.
My young chickens figured it out in one day, while they appear to be dumb, they have instinct and when it comes to water they figure things out really fast lol!