Liquids can rise over the crest of a siphon because they are pushed by atmospheric pressure. Siphons must be started by filling them in any number of ways. After priming, atmospheric pressure acts on both ends of the siphon, but the longer leg carries a greater weight of liquid. Gravity then drains the liquid through the longer leg, and this maintains the low pressure that was established at the start. Capillary action can enhance the siphon and cavitation may modify the phenomenon and cause the siphon to 'break'..
Once started, a siphon requires no additional energy to keep the liquid flowing up and out of the reservoir. The siphone will pull the liquid out of the reservoir until the level falls below the intake or outlet of the siphon, whichever comes first. Energy is conserved because the ultimate drain point is lower than the liquid level of the reservoir.
The maximum height of the crest is limited by atmospheric pressure, the density of the liquid, and its vapour pressure. When the pressure exerted by the weight of the liquid equals that of atmospheric pressure, a vacuum will form at the high point and the siphon effect will end. The liquid may boil briefly until the vacuum is filled with the liquid's vapour pressure. For water at standard atmospheric pressure, the maximum siphon height is approximately 10 m (33 feet); for mercury it is 76 cm (30 inches).[dubious – discuss]
An analogy to understand siphons is to imagine a long, frictionless train extending from a plain, up a hill and then down the hill into a valley below the plain. So long as the valley is below the plain, the part of the train on the valley side of the hill will be longer and heavier than the part on the plain side of the hill, so the portion of the train sliding into the valley can pull the rest of the train up the hill and into the valley. What is not obvious is what holds the train together when the train is a liquid in a tube. In this analogy, atmospheric pressure holds the train together. Once the force of gravity on the couplings between the cars of the train going up the hill exceeds that of atmospheric pressure, the coupling breaks and the train falls apart. The train analogy is demonstrated in a "siphon-chain model"  where a long chain on a pulley flows between two beakers.