It took a few tries, but this Pythagoras' Cup really does work. A hole near the bottom leads to a tube that loops up the side of the cup, then back down to an open end. If you fill the cup too full (above the level of the top of the tube), the tube acts as a siphon, drawing all the liquid out of the cup and spilling it out the open end. Check out the video!
Note: I would guess that the glaze doesn't cover 100% of the surface inside the tube, and it's the glaze that's food-safe, so you might not want to drink out of this.