I have to tell you, I wish I could open up the heads of some parents and drop a little bit of my grandmother's common sense into their brains.

For example, I recently visited a high school to talk to some teens about money management. Right in the middle of my presentation, some kid's cell phone rang. He jumped up and left the room.

"Wait," I said as he headed out. "Who is that on the telephone and why is he or she calling you while you're in class?"

"It's my mother," the student snapped at me. "I have a doctor's appointment and she's calling to remind me."

Mind you, he didn't need to leave for the appointment right then.

The student apologized on returning to the classroom after taking the call. But frankly, I blame that boy's mama for his rude behavior.

All the boy's mother needed to do was send a note with him to school and tell him what time she was going to pick him up. If he wasn't outside when she got there, she could go into the school and have him called to the office.

That episode got me wondering why so many parents have bought into this idea that their child needs a cell phone.

It seems cell phones are as prevalent among preteens and teens as acne.

About one-third of 11- to 17-year-olds in the United States have a cell phone, according to Yankee Group, a Boston-based research firm. By 2007, nearly half of that age group will have cell phones, the firm estimates.

Folks, that could amount to billions of dollars of your money going to wireless carriers just so your child can chat with friends, download ring tones or play games.

I have a cell phone, but I don't need one. It's true that it's harder these days to find a public phone. And some people may actually need a cell phone (doctors on call, emergency personnel). But let's be honest. Most of us use these pocket-size phones to conduct conversations that could wait until we got home or to the office.

I know some parents argue that they're buying the phones because they want to stay in contact with their children. They say the phones are for an emergency -- for the child's safety. Oh, sure.

It is more likely that the teen or preteen -- egged on by masterful marketing and peer envy -- whined for a cell phone and some adult just caved. The reality is that cell phones are used 99.9 percent of the time by non-business consumers for idle chatter.

"This is a case of the consumption of time and money that could be going elsewhere," said Wanda Urbanska, host of the upcoming PBS series "Simple Living."

The average preteen or teen's cell phone bill is $45 a month, according to Yankee. Just so you know, preteens and teens who have their own cell phones generate higher monthly bills ($50 or more a month) than those who share a cell phone with another member of the household ($38 a month).

So, using the $45 average monthly figure, let's say you get your son a cell phone on his 13th birthday. By the time he graduates from high school at age 18, you would have spent at least $3,200 on monthly cell phone charges.

Let's pretend you saved that money instead. What could your graduating high school son do with $3,200 (not including any interest or investment return you might have earned)?

That money would make a nice down payment on a car (I suggest a used one). He could use the money for the security deposit on an apartment and to buy some furniture.

That amount of cash could easily pay for his books and supplies as a college student.

"It's important to aggregate this kind of expense over time, but people often don't look at it that way when they get something like" a cell phone, Urbanska said.

According to a recent study by the JumpStart Coalition for Personal Financial Literacy, high school seniors are failing to understand the basics of personal finance. On average, students who participated in the 2004 survey answered 52.3 percent of the questions correctly.

I learned to be a smart consumer from my grandmother. She never would have allowed me to have a cell phone.

In fact, Big Mama wouldn't buy me stuff even when I knew she could afford it.

"Child, I don't want you to have it better than I did," she would say.

That sounded selfish and nonsensical to me then.

But Big Mama had a point.

"If I give you everything you want or what your friends have, then how will you learn to live on what you make?" Big Mama would ask. "How will you learn to appreciate the things that really matter?"

Think about it this way. Kids will nag you for candy or junk food all day, every day. But you know what they need is a healthy, balanced meal. So you say no -- over and over again. To do otherwise is not good parenting.

But why is that lesson often lost when it comes to making consumer purchases for children? Just because marketers say something is a necessity doesn't make it so.

"Parents have to look at things that have become the norm in contemporary life and question whether it's wise," Urbanska said.

I agree. Take a cue from Big Mama and make your preteens and teens hang up those cell phones.

Researcher Lorraine Denis-Cooper contributed to this column.

Michelle Singletary discusses personal finance Tuesdays on NPR's "Day to Day" program and online at www.npr.org. Readers can write to her in care of The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or send e-mail to singletarym@washpost.com.