Overflows?

[instinkt]

I'm just curious how external overflows work. I understand the basic idea, but how does the siphon become primed?
I've tried searching around for the answer, but all I can find is a basic explenation that the water reaches a certain level then flows over and out to a skimmer/sump, but what I don't understand is how the water is sucked up and over the tank wall in the first place?
Anyone care to shed some light?

[dattack]

The basic overflow requires you to suck out all the air in the U-tube while turning on the pump to make it run.

[clamster]

Use a very long air tube! But it's just me, I don't like the taste of salt in my mouth very much :D

[bsempel]

water always flows downhill.

since the water level in the exterior box is below the level of the interior box the water will syphon once it is started.

to start, as stated, sucking the air out of the overflow tube will begin the water "falling" into the exterior box.

hope that makes sense.

[instinkt]

Yeah thanx guys. I understand the concept of siphoning. I just didn't know if the siphon was somehow automatically started or not. But yeah I guessed it would have to involve some kind of manual air removal :)