Whether you need to get water out of a fish tank or gas out of a car, it’s useful to know how to siphon liquids.

Siphoning is simple physics. Contrary to what many assume, a siphon does not rely on atmospheric pressure (good news should you ever need to siphon gas from your spaceship) but rather gravity (a potential problem for the spaceship scenario). Due to the cohesive forces at work in a column of liquid, once you start a siphon, it’ll keep going on its own. Just make sure to cut it off when the desired volume has been transferred.

What You'll Need

To start, you’ll need the source of liquid, a place for the liquid to go, and a tube or hose. A clear tube adds the fun element of being able to see the liquid as it is being transferred, but this isn’t mandatory.

In order for the siphon to work, the source liquid must be elevated above the container you're trying to transfer it to. Remember, gravity is doing the work here.

Getting Started

Feed the hose into the source tank and put your secondary container on the ground. Next, you’ll need to get rid of the air in the tube by sucking out the fluid from the source tank. You can do this with your mouth, but this is a bad idea if you're siphoning gas. A siphon pump will do a better job than your mouth, and it’ll eliminate the chance of ingesting something toxic. This simple device usually costs less than $10. To avoid air bubbles when sucking out the liquid, hold up your siphon tube vertically—this will give the bubbles a place to escape.

If you want to skip sucking altogether, submerge the entire hose in the source tank, then place your thumb firmly over the delivery end. While holding the delivery end of the hose closed, move it to the receiving vessel. Make sure the other end is still submerged, then remove your thumb. If you've done it right, the siphon will flow.

Letting it Flow

Once the air is out and the liquid has reached the end of the tube, you must prevent any air from getting back in. To do this, maintain suction and carefully crimp the hose or use your thumb as a stopper. Now drop the end of the hose into the other container and release. Liquid should start traveling from the source container to the new one.

Be sure to keep an eye on your source liquid and make sure the hose stays fully submerged, otherwise you'll end up with bubbles.

Stopping the Siphon

When you need to stop, lift the new container and hose higher than your source container. Then, remove the hose and let the excess fluid in the hose drain back into the source. Or, if you're draining something large like a fish tank or Jacuzzi, lift the hose out of the source.