Professor James Roberts remembers when he used to walk into his Baylor University classroom and have to to yell over the din of his students, begging them to quiet down for the start of class.

Today, he says, he walks in the room, "and you can hear a pin drop."

They are all on their phones.

To any parent of teenagers like Roberts - actually, to anyone who spends time in public places in America - this is probably not too surprising. As our smartphone dependency becomes ever more socially acceptable, the number of places you won't find people glued to palm-sized screens is dwindling.

So why would a major charity base one of its largest fundraisers on the hope that people would put down those phones?

That's the gamble UNICEF took last year when it adapted its Tap Project (donate $1 at restaurants for your free glass of water) to a pitch asking potential benefactors to set their phones down on a table, and leave them there.

A special app then counts every second they can go without touching their phones. For every 10 minutes, an outside donor had pledged to UNICEF the funding for one day of clean drinking water for a child in need.

It worked. In a month and a half, more than 2.6 million people spent nearly 255 million in the "phones down" mode. In return, Giorgio Armani Fragrances and other donors contributed more than $1 million to UNICEF's water and sanitation programs. The average time users spent without their phones was an hour and a half.

This year's Tap Project (accessible on your phone at tap.unicefusa.org) begins on March 1.

UNICEF president and CEO Caryl Sterns said that people will agree "to do without something we think we can't do without, in order to ensure others get something they really can't live without."

Something we think we can't do without. How did we get to this point anyway? Roberts, the professor, put his classroom observations to the test when he tried to figure out just how much college students use their cell phones. His study found that male college students spend eight hours a day on their cell phones. Female students spent 10 hours. Sixty percent of his participants admitted they might be addicted to their devices.

"While they are watching TV and doing their homework, they are posting pictures to Instagram, tweeting and answering emails," he said. "It's easy to throw in nine, 10 hours a day when you are multitasking."

Adults and professionals often feel that same pull - that need to check their work email after-hours, say.

"In a psychological sense, we feel like, 'everyone expects me to be on,'" said Leslie Perlow, a Harvard Business School professor who wrote the book "Sleeping With Your Smart Phone." "And you're right, everyone does expect you to be on."

Perlow's research has shown that for many people, stepping away from their phones doesn't seem like a freeing experience. It gives them anxiety. They might be inclined to take a technology breather only if they have a legitimate reason to do so, like a planned vacation or an agreement from their boss that they aren't expected to answer emails past 7 p.m.

The Tap Project's success, Perlow said, might be the notion that helping a charity gives them an excuse to unplug.

It's also a way to contribute to a good cause without actually opening their wallets.

"We come from a place of believing fundamentally that everyone wants to help, wants to do good," Sterns at UNICEF said. "We're just reducing the friction."

It took the charity three years to land on this successful mobile experience. They previously tried to convince people to send "virtual water" on Facebook and bid in an online raffle for tap water from the homes of celebrities.

Now, they are convinced that as much as people rely on their phones, they will walk away from them if the moment is right. You can try it while you're reading a book, or at dinner with friends - or, as is the new standard in the UNICEF offices, while at work. Once the project begins, Sterns said, she's going to have a hard time reaching any of her staff.