Fifteen-year-old Shannon Kelly and her brother used to text back and forth between their bedrooms at night, arguing over who was going to turn the light out.

"You turn it out."

"No, you turn it out."

When the $200 phone bill arrived at her Bloomington home, she lost her cell phone. She's hoping to get it back, along with unlimited text messaging, for her 16th birthday this month.

"I need my unlimited texting," she said with a laugh, sitting at her high school cafeteria lunch table.

Apparently, a lot of other teens need it too. It's the digital version of a folded note being passed under a desk, a way to silently share gossip, make plans, let a parent know the bus is late or tell dad what the score is.

Zach Lauer, a Central Catholic High School baseball player, updates his dad from the dugout.

"He can't be there so he texts me a lot," he said. "I send him the score."

Although parents may struggle with the shorthand messages, they're starting to get the hang of it and wireless phone companies hope they do, adding their 10 cents, or a whole lot more, to the billions of messages being exchanged.

Sixty percent of teens use text messaging, according to a recent HarrisInteractive poll. In the first quarter of this year, Verizon Wireless tracked 22.3 billion messages, more than double the number in the same period in 2006.

"Everyone has these preconceived notions that it's just for kids, but I see parents more and more using it," said Carolyn Schamberger, Verizon spokeswoman. "It's an affordable, unobtrusive way to be in touch with their kids."

Unless you're on an unlimited plan, charges for messages sent or received can quickly add up.

Sixteen-year-old Michael Ginnetti of Bloomington sends about 40 messages a day, usually going 200 or more over his 1,000-per-month limit. For every one he's over, it costs him about 5 cents. He's lost his phone for a week too, after being caught talking at 12:30 a.m.

"That was really rough," he said. "But that was after they told me not to talk on it."

Since texting can be done covertly, even under a sweatshirt with teens who've memorized the keys, texting after hours can be particularly difficult to detect.

But parents can track activity through phone records, almost in real time if they monitor an account online.

The teens admitted to texting under the table and while driving.

"You get good at it," said 16-year-old Brittany Brady of Bloomington. "It's like glancing at the radio."

The users were unanimous on when it wouldn't be appropriate to text - to break up with someone.

"That's a no-go," Ginnetti said.

And that's consistent with the poll of about 1,000 13- to 18-year-olds, which found that only 3 percent would text a break up and 4 percent would argue over a tiny phone screen.

Although parents may not understand why teens would text rather than simply pick up a phone, it's perfectly normal for those who grew up with the Internet, said Anastasia Goodstein, the San Francisco author of "Totally Wired: What Teens and Tweens Are Really Doing Online," which came out in March.

"They like talking. It's just one more way to communicate. How many times do colleagues use e-mail when they should get up and say something in person?"

She wrote the book to make parents less anxious, to remind them of their teen years, when they'd walk in the door and pick up the phone to call a friend they'd been in school with all day.

"Remember when you'd be watching TV with the phone in your ear and you'd only say two words?"

Teens seem to know when it's best to switch to other forms of communication, the poll found. Ginnetti agreed, saying it's the little stuff that gets sent in a text message.

"If it's news, someone would call me."

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Decoding

Parents: This may seem 2G2BT (too good to be true) but all you have to do to stay up on shorthand lingo is log onto Teen Chat Decoder, an online acronym database. Key in the acronym or abbreviation and it decodes it. Go to www.teenchatdecoder.com. Here are some examples:

• POS: Parent over shoulder

• PIR: Parent in room

• PAW: Parents are watching

• P911: Parent alert

• 2MI: Too much information

• BF or GF: Boyfriend or girlfriend

• ASL: Age/sex/location

• RUOK?: Are you OK?

• OTP: On the phone

• GIAR: Give it a rest

• GOI: Get over it

• ROTFL: Rolling on the floor laughing

• JMO: Just my opinion

• BFF: Best friend forever

• HAK: Hugs and kisses

• OMG: Oh my God

• NW!: No way!

• WFM: Works for me

• WYRN: What's your real name?

• MUSM: Miss you so much

• TTYL: Talk to you later

• C-P: Sleepy

• CUL8R: See you later

• H2CUS: Hope to see you soon

• UGTBK: You've got to be kidding

SOURCES: Webopedia.com, netlingo.com