TBR Physics II page 99 TBR says that a toilet uses a siphon and that "in order to have water flow from the intake end to the output end, there must be a net pressure difference, where the gauge pressure is greater at the output end" Can someone explain the bolded? Why does pressure have to be greater at the output end? This is not intuitive to me. For example, lets say you have 2 adjacent rooms at different pressures (e.g., 1 ATM, 1.1 ATM). When you open the door between the rooms, doesn't air flow from high to LOW pressure?