Great question! I was wondering this exact same thing when cleaning a
fish tank a while ago, and the answer is a lot simpler than it seems.
The device that you're talking about is basically just a long hose.
(This is actually the same thing as if you use a hose to siphon gas out
of a car.) You fill the hose up with water, then put one end in the
fish tank and the other end in a bucket next to the fish tank. What
happens is that when you hang the hose over the edge of the tank, the
water starts to fall out of it in both directions. But the submerged
end of the hose has water pressure pushing back on it, so the water on
the other side falls out much more easily (into the bucket).
When the water falls down into the bucket, it lowers the pressure
in the hose. Then the higher pressure water in the fish tank pushes
into the hose, the same as the way water goes up in a straw when you
lower the pressure on the top by sucking. You can think of it this way:
if the tank water didn't follow along, you'd have a vacuum left in the
hose- and you know the vacuum would suck water out of the tank. When
the water leaves the hose, it's lower than the water level in the tank,
so the net effect is just to have water falling.
As for your other comment, you don't actually have to use your
mouth to get this started. All you have to do is get the hose
completely filled up with water. You can do this by submerging the
entire hose in the fish tank to fill it. Then put your fingers over the
ends and move one end into the bucket.
On the other question you raise- does water stick together- the
answer is yes, some. Just like you say, if it stuck together much more
it wouldn't break up into lots of little drops when you spill some. But
it doesn't just spread out all over the place as a smooth layer. It
beads up, because it does stick to itself some. That's the same effect
(surface tension) that makes it possible to slightly overfill a glass
of water. I'm not sure, but it seems like that helps keep the flow in
your siphon from breaking into counterflows of air and water, which you
could imagine happening in a big tube, and which would keep the siphon
(republished on 07/31/06)