Gas pumps don't work when the people who run electric power grids have been consumed by the living dead. Learn the principles behind siphons and how to siphon gas out of abandoned cars by the side of the road.

Siphoning is one of the neater physics visuals that one can create with simple supplies. All you need to siphon is: a container of liquid, a long, flexible tube, and a mouth strong enough to suck the liquid through the tube. The mouth doesn't have to be your own. All you need to do is suck the liquid from the container over the top edge and down, until it starts running freely. (At this point you also need another container if you don't want the liquid to spill anywhere. Should I have told you that earlier?) The water will continue to run until the liquid level between the two containers (Have you put one at the other end of the tube, yet?) equalizes or air gets into the tube.

Many people believe that siphons work due to atmospheric pressure. This is probably because they looked at its definition in the Oxford English Dictionary in the last 99 years. All that time, the dictionary has described the force that propels the liquid from one end of the siphon to the other as atmospheric pressure, until a physicist wrote in and corrected them.

The force is, in fact, gravity. The mechanism of this has been described in many ways, even on the ‘Ask a Scientist' section of the Argonne National Laboratory website. The easiest way to understand is to forget that the liquid is a collection of separate molecules. If the liquid were beads on a string, and it were pulled over the side of the container so much that the weight on the siphon side of the container exceeded the weight on the other side, the beads would begin freely falling out of the container. They would continue to do so until the bead level on either side of the container equalized, making the pull of gravity on either side of the container exactly the same.

As it turns out, though, atmospheric pressure does play a part in most siphons. Atmospheric pressure, and the cohesion of liquids, both play a part in making sure that liquid keeps flowing through the siphon. These are the string that holds the beads together. If the siphon is strained too much, the little vacuum – or gas – pockets open up in the flow of water. This snaps the ‘string' and stops the siphon from working.