The Trick to the Archer Fish's Powerful Attack To capture their prey - a spider or insect, often sitting on branches above the water - archer fish spit out a powerful jet of water. The jet knocks the prey off and the fish race to capture their dinner. "When the jet hits a solid surface you can hear a loud knock that's very impressive because the fish are very small," said Alberto Vailati, a physicist at the University of Milan. Just how the fish generate a stream of water powerful enough to detach prey from vegetation has been a mystery. Now Dr. Vailati and his colleagues say the fish are able to modulate the velocity of the water coming out of their mouths. They increase the velocity over time and the water builds up as it is spit out in a continuous stream. "You have this drop of water that progressively increases in size over time, and gets inflated," Dr. Vailati said. This large drop of water is then able to hit and knock down prey with incredible force. Dr. Vailati and his colleagues studied the fish, which are found in South and Southeast Asia, Australia and Polynesia, with a high-speed video camera. Previously, researchers had speculated that archer fish had some sort of special muscle in the mouth to enable the powerful jet of water. That hypothesis was disproved. The technique used by the fish resembles something called water jet cutting, Dr. Vailati said, in which jets of water cut through metal. He and his colleagues report their findings in the current issue of the journal PLoS One.