When a group of neighborhood kids have a water fight, the amassed weaponry is truly astounding. An aquatic arms race has resulted in Super Soaker water guns that can blast streams of water fifty feet across the yard. Water bazookas, water canons, motorized water machine guns--times certainly have changed from the days of the simple, see-through plastic water pistol.
But the basic principles of the water gun have remained the same. If you think back about a decade, you'll recall that the basic, standard issue water pistol consists of a few main parts: a plastic casing that acts as a water reservoir, a trigger connected to a piston and spring, and a plastic tube to draw water from the reservoir into a cylinder, at the end of which is the gun's nozzle.
Because they are usually made of clear plastic, you can watch the gun in action. In effect, a water pistol works on the same principle as a water pump. When you pull the trigger and the piston pushes it in, it decreases the volume inside the gun, which forces water out through the nozzle.
You're basically pumping water out of the gun. When you release the trigger and the spring pushes the piston back out, there's more space inside the gun, which sucks more water up from the reservoir and into the pump. Small, one-way valves keep the water flowing in only one direction: up from the reservoir, into the pump, and out through the nozzle.
Super Soakers use a complex system involving large pumps and expanding bladders. But these modern water weapons owe everything to the simple beauty of the original water gun.