The surface of water placed in a single container (a glass or a bathtub or a lake) will remain at the same level relative to Earth on both sides of the container. Adding water to one side will only make the entire level uniformly rise; there can never be one section of the glass or tub or lake that is at a higher elevation than another section. To understand this fact, consider adding the small cube of water on top of the surface at one location. It would exert a downward force on the water under it. But, because water can flow, water under it would flow outward, raising the level elsewhere in the container until the pressure is equal everywhere.

Water also seeks its own level in other containers. If you fill a hose or tube with water and hold the tube in a “U” shape, the water level will be at the same locations in the two ends. You can use the “U” tube to make a device to show you equal heights at two different locations. You may have a coffeemaker that has a water height indicator on the side. This is a small tube that connects to the water reservoir at the bottom. The water level in the narrow tube and the wide reservoir is the same.