A few funiculars have been built using water tanks under the floor of each car that are filled or emptied until just sufficient imbalance is achieved to allow movement. The car at the top of the hill is loaded with water until it is heavier than the car at the bottom, causing it to descend the hill and pulling up the other car. The water is drained at the bottom, and the process repeats with the cars exchanging roles. The movement is controlled by a brakeman.