Water guns are the perfect toy in the summer. You can pump one up and blast your friends. All water guns are pumping machines. Inside, they have tubes, valves and pistons designed to move water around and pressurize it. Here's what we found when we cracked this one open:

A big water container - a reservoir - sits at the back of the gun. This is where you pour in the water. The reservoir is connected to a long narrow cylinder, with a moving piston inside it. This is the pump. When you "pump" the gun, you move the piston back and forth in the cylinder.

A flexible tube connects this pump to a hard tube at the top of the gun. The hard tube has a nozzle on one side and a thick rubber balloon on the other end.

A one-way valve sits between the pump and the reservoir, and another one-way valve sits between the pump and the tube leading to the top of the gun.

A one-way valve is a spring-loaded plunger sitting in a water passageway. Normally, the spring holds the plunger against the passageway, keeping the passageway closed. If you push water one way through the valve, the pressure will push the plunger against the spring to open the passageway -- water flows in that direction. But if you try to move water the other way, the pressure will just push the plunger against the passageway opening, and no water can flow through.

In this gun, the first one-way valve lets water flow from the reservoir to the pump, but not from the pump to the reservoir. The second one-way valve lets water move from the pump to the top of the gun, but not from the top of the gun to the pump.

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So what's going on here?

When you pull the piston out, it "sucks" water from the reservoir, through the first one-way valve. It works the same basic way as a straw: The cylinder is airtight, so when you pull the piston out, there is a vacuum in that area. In other words, the air pressure drops, creating a high-suction area. This suction is strong enough to move water out of the reservoir into the pump.

When you push the piston in, it forces water out of the cylinder. The first one-way valve stops the water from flowing back into the reservoir. Instead, the water flows through the flexible tube to the thick rubber balloon.

As you keep pumping, you fill the balloon up with water. The balloon exerts a lot of pressure on the water, but the water has nowhere to go until you open the passageway to the nozzle by pulling the trigger.

When you pull the trigger back, it pulls a sliding plastic piece, which pulls a metal rod connected to a plastic lever. The plastic lever pulls back a plunger, opening the passageway to the nozzle. The pressure from the balloon forces the water through the nozzle, in a steady stream.

Here's the step-by-step breakdown:

1. You "pump" the gun by pulling and pushing on the piston.
2. As you pull the piston out, it "sucks water from the reservoir, through the first one-way valve.
3. When you push the piston in, it forces water out of the cylinder and into the thick rubber balloon. (The first one-way valve blocks the water from flowing back into the reservoir.)
4. As you keep pumping, you fill the balloon up with water.
5. When you pull the trigger back, it pulls a sliding plastic piece, which pulls a metal rod connected to a plastic lever.
6. The plastic lever pulls back a plunger, opening the passageway to the nozzle.
7. The pressure from the balloon forces the water through the nozzle, in a steady stream.