The doorbell rang and Nick Hine pulled back the slide on his yellow Nerf pistol, drawing a foam dart into firing position before answering the door at his Wilmette home.

This is what life has become for about 170 New Trier Township High School seniors playing Paranoia, a game that pits crews of assassins armed with toy guns against one another on the not-so-mean streets of the North Shore.

In a show-no-mercy contest that mirrors similar matches under way in Gurnee, Naperville and on college campuses, players have lurked in bushes, fired from cars — even pretended to sell Girl Scout cookies to sneak up on someone and make a hit.

The game has both entertained and startled parents, who have encountered strangers at their doors, in their kitchens or running down the basement stairs to assassinate a son or daughter.

Their complaints about the so-called "naked rule" — players in the buff weren't allowed to be shot — recently led to a modification in the rules of engagement.

Organizers had intended to protect participants from being surprised while taking showers, but one young man actually stripped so he could chase his foe down the street, Nerf gun in hand, without worrying about being gunned down himself. After several reported sightings of naked players, that is now a no-no.

"A few (nude) people would lie in wait in friends' cars and then jump out, then pursue their adversary," said Sam Hine, 17, twin brother of Nick and one of three "commissioners" overseeing the game. "We eventually closed off the loophole so that if you don't have clothes on you can't be killed, but you can't kill other people, either."

As the game progresses, blogs and Facebook entries document the most sensational kills with vivid detail. Teens have taken to the streets, suspiciously looking over their shoulders as they walk home, work out at the gym or head to work.

Rules, shaped by the game's organizers, forbid game-playing on school grounds, during class times or at school-related events or church. In Gurnee, the rule also bans workplace play.

"If you work at Jewel, you can't just start shooting back in the produce aisle," said Ben Niskanen, 18, a senior at Warren Township High School. "And you can't break into … houses because we don't want people getting arrested."

The New Trier competition calls for each participant to pay a $5 entrance fee. Players form teams of five members who every week are assigned to "kill" everyone on a randomly selected opposing team. Once someone is shot, they are out of action for the rest of the week. The game resumes at 12:01 a.m. every Monday and ends on Sunday at 11:59 p.m.

After seven weeks, the top-ranked teams — with names like Blood, Bath and Beyond and the Jew Crew — will face each other in a bracket-style tournament to determine the champion. The game, in its second month, will resume after the holiday break. The winning team divvies up an $800 prize.

"It really changes the way you view the entire township," said Sam Rodman, an 18-year-old New Trier senior. "We have jocks, nerds, theater kids, hipsters and they're all playing Paranoia."

Nick Hine, a lanky teenager in preppy clothes and fashionably shaggy hair, recently woke up at 5:45 a.m. to stake out a classmate's home in Glencoe. He trailed the target to school and confronted him as he got out of his car, only to have his gun jam.

Players are required to tell their parents about the game so there are no surprises or unintended consequences.

One student was suspended from the game after he leapt into a moving truck during play. Another was disciplined after he tried to assassinate a competitor on a field trip, said David Berlinghof, 17, another commissioner.

One blog entry labeled "Parental concerns" asks the teens to use good judgment and reminds them that they must obey the laws.

"Remember, there are ways to enter someone's house without necessarily breaking the law, which you should never do," the blog states. For example, "A New Trier junior girl dressed in a Girl Scout outfit and rang a doorbell, purporting to sell cookies to her target. The parents summoned the unsuspecting victim, who was quickly neutralized."

In Gurnee nearly 30 people from Warren Township High School began playing a few weeks ago, said Kyle Alden, 18, a senior.

Alden has made two kills and survived early rounds, he said. When a reporter contacted him for this story, his first thought was that it was some kind of elaborate scheme that would leave him exposed.

"It definitely makes you paranoid in a lot of aspects," said Alden.

Alden holds team meetings and strategy sessions, but insists it's all in good fun. Since he started playing, he hasn't kept his regular gym schedule — routines can be deadly.

Lest someone hit him on his way out, he wakes at 5:30 a.m. to pull his car into the garage after his dad pulls his car out.

"I don't want to risk it," he said.

In Naperville, Paranoia was in full swing about a year ago. This year, 20 to 40 teens gather to play for a few hours at a park or empty house, said Justin Boswell, 18, a senior at Naperville North High School.

They are still out for the thrill of a clean kill, but the game doesn't spill beyond the allotted time and location and the winner's pot of money is no longer the big draw.

The best part?

That nostalgic feeling of what it was like to get your first Nerf gun as a child, Boswell said.

"As people are talking to us about growing up, it's nice to step back and be a kid again," he said.

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