Ready, set, squirt! How one inventor used charts and graphs to make a splash.

By day, Lonnie Johnson worked as an aerospace engineer. By night, as his wife and kids slept, Johnson tinkered with inventions he hoped would radically improve people's lives. Late one weekend in his bathroom, Johnson experimented with a new refrigerator cooling system--it would run on water and reduce air pollution. But when he tested the contraption's cooling hose, it blasted water into the bathtub and sent the shower curtain flying. Wow, Johnson thought, this could be the ultimate water gun! That brainstorm changed his life--and maybe made yours more fun.

Until then, squirt gun science was a no-brainer: Pull a trigger and you force water in a chamber out through a narrow opening--the same principle at work in perfume bottles and household cleaners. Johnson's inspiration: A squirt gun that stores energy could blast water like a powerful garden hose.

The result was the Super Soaker--a sale but high-pressure water pistol that volleys [H.sub.2]O up to 15 meters (50 feet). It took the engineer seven years to develop his toy (it was first mass-produced in 1989), but today industry experts call it the world's most popular squirt gun. How did Johnson do it?

SUPER SOAKER PHYSICS

Johnson built a prototype (rough design) of his idea--an empty soda bottle strapped onto plastic PVC pipe. "The first gun I made blasted Styrofoam cups off a table," he says.

The design relied on two fundamental principles of physics: force and pressure. He knew your basic squirt gun's force (push of pull on an object) is limited to the strength of your trigger finger. Each squeeze of the trigger creates pressure (force applied over an area) on the water inside the gun, shooting it out the nozzle. To drench a friend, you'd have to squeeze the trigger over and over again. To top it off, without enough pressure, the water doesn't have enough velocity (speed) to jet very far.

The Super Soaker packs a punch because you pump the gun with your whole arm and the gun stores the energy. In physics lingo: The mechanical energy from your arm transfers to the gun and becomes the kinetic energy of the water. Each pump sends water from the gun's reservoir into an air chamber. As more water pumps in, more air is trapped and squished, or compressed. Just pull the trigger and all that compressed air is released at once in a liquid blast. "Air stores energy like a spring," says Johnson. "The more you compress it, the more it bounces back."

The Super Soaker's nozzle also figures in the toy's success. The smaller a nozzle's diameter (distance across a circle), the more pressure it creates to spray a distant target. But the stream sprays so fast it breaks into droplets before it reaches the target. Johnson's secret? "It comes down to water holding together as a stream. It maximizes the amount of water you can drench somebody with."

LINE IT UP!

In honing his invention, Johnson faced a design dilemma: Water shot from a wide nozzle sticks together, but a narrow nozzle makes for a better firing distance. To find the perfect nozzle size, Johnson experimented with how water sprays from four different nozzles--each with a different diameter.

First he created a data table. A data table includes all the variables (characteristics being tested) in an experiment and helps you organize information efficiently. "The data paints a picture for you," Johnson says.

Next, Johnson used a machine that blasted water through each nozzle at exactly 60 psi (pounds per square inch) of pressure--about the same amount of pressure pumped by an average 10-year-old. This guaranteed any changes in the spray would be due only to the size of the nozzle. He measured how far the water squirted from each nozzle. And he observed how much the stream stayed together. A rating of 1 meant the stream splattered like a sprinkler; a rating of 5 meant the stream scattered very little--like faucet water. Johnson recorded the results of each trial in a table, similar to the one below.

Results in hand, Johnson still didn't have the answer he needed. Plotting two line graphs makes it easy to see how nozzle size affects the gun's range and splatter rating (see below). But what if the perfect nozzle size was not one of the four he tested? A line graph plots all points in between variables. When both graphs overlap, the intersection (point where the two lines cross) suggests the perfect nozzle size. End result: the ultimate water gun to soak your friends from a safe distance.

DATA TABLE

KEY:
D = Distance (meters)
S = Splatter rating (1 = bad splatter, 5 = good splatter)

NOZZLE EXPERIMENT RESULTS *

Nozzle   TRIAL 1    TRIAL 2    TRIAL 3    AVERAGE
Size     D     S    D     S    D     S    D     S
(in.)

1/32     25   1.9   26   1.9   24   1.6   25   1.8
1/16     19   2.9   19   3.1   19   2.7   19   2.9
1/8      13   3.1   11   3.3   11   3.2   12   3.2
1/4      9    4.9   7    4.9   8    4.8   8    4.9

* Data are approximated.

QUESTION: Compare the results of the three trials above. Why is it a good idea to conduct more than one trial of an experiment?

SUPER SOAKER SCIENTIST

Before he dreamed up the Super Soaker, Lonnie Johnson worked in NASA's Jet Propulsion Lab where he helped design the Galileo space probe. But invention is his passion. Here, SW grills Johnson about his life as an inventor:

WHAT WERE YOU LIKE IN JUNIOR HIGH?

I wasn't preoccupied with going out and having fun. I was pretty focused on doing stuff I was interested in. I was in science clubs. And I used to launch bottle rockets at school--something you couldn't do now.

WHAT WAS YOUR BEST SCIENCE PROJECT?

I won a state science award for building Linex, a robot, when I was in high school. He was about 4 feet tall, remote controlled, and operated on compressed air--like the Super Soaker. Ha had arras and could pick up stuff, hold things, and more around.

WHAT'S THE SECRET TO A GOOD SCIENCE PROJECT?

One of the most important steps in the scientific method is observation, because you get hints of what you're looking for. You have to really think about it and make changes based on what you see.

WHERE DO YOU GET YOUR INSPIRATION?

Need is the mother of all invention. If there's a problem that can be solved, that's a good place to start looking for a project.