This will probably be obvious to most of you so my apologies in advance. Some classmates and I got into a discussion about how an automatic feeder like this (http://i.ebayimg.com/t/Auto-Refill-...QCGk~$(KGrHqEH-C0EtrFJ6qq-BLdHF((cVg~~_35.JPG ) would work. When thinking about it I had guessed that it may be similar to hydraulics where the high pressure of the column would be countered by a large surface area in the pool (where a classmate had suggested atm pressure would be part of the opposing force here). Other classmates suggested that when the water level is low enough air can displace water (essentially creating a vacuum in the bottle). Can anyone shed light on the actual physics at play here? Your time is much appreciated.