Hello. I saw this video: ... it is about water pump. I want to know how it works. First straw is above the water surface, second is below the water surface. To the straw below the water surface goes watter because force of gravity act on it. This is why air column increases and makes it underpressure. So, the underpressure acts to the water in first bottle and sucks the water to maintaining atmospheric pressure. It is right? However, there is one think which I really don't understant. Why the first glass must be higher than second glass?