Why can't I swallow the entire (liquid) contents of my mouth at once? I've tested this out using water - I manage to fit a certain amount of water in my mouth, but if I try to gulp it all at once, I fail: I feel like spitting the water out (gag reflex?) There's a certain limit, say 60%, of the full amount of water, that I can insert into my mouth and then gulp it without drowning myself. Why is that? What sort of muscle coordination causes this?

Normal swallowing triggers a peristaltic motion towards your stomach.

However, as demonstrated by young "scientists" (search "water chugging tutorial"), one can train to voluntarily keep the esophagus valve open and poor down liquids without triggering peristaltic waves.