Fitted through the cap is a stiff suction tube made of clear plastic. With the suction bottle you can effectivly suck up all your gold from the goldpan. The suction tube has an inner diameter of 3 mm.
The suction is so strong that you can even suck up small nuggets. All the gold that is sucked into the bottle, stays safely there until the cap is unscrewed and you empty the bottle.
Every gold miner in the world uses this.