Geologist Phil Christensen had traveled 14 hours to Iceland just to collect a few rocks that had formed in the volcanic caldrons of that island nation when he noticed a school "literally across the street" from the rocks he was picking up.
It was, he said in a telephone interview, one of those "ah ha" moments that come so rarely, even in the life of a scientist.
"I thought, I could have just called and asked those kids in the school to send me some rocks," he recalled. "I could almost see a light bulb going off in my head."
Not long after picking up his Icelandic rocks that formed in conditions thought to be similar to how many rocks are formed on Mars, Christensen was on temporary duty at NASA's Jet Propulsion Laboratory in Pasadena, Calif., serving as principal investigator for mineral-detecting spectrometers on NASA's two Mars rovers, Spirit and Opportunity.
As a major player in NASA's Mars missions, he routinely took part in televised press conferences, so one day he casually mentioned that it would be nice if kids would send him some rocks.
The request shot around the world on NASA's television network, prompting a collective groan from his 50 employees at his home base, Arizona State University in Tempe.
"They didn't know anything about this," he recalled. "So here I am on this press conference saying, 'hey, send us your rocks.' Everybody in the room (at ASU) yelled, 'no, don't say that Phil, don't you understand the power of the Internet?'"
"Three days later, we got our first rock," Christensen said. "The next day it was three. The next day it was eight, and the next day it was 17. Within about two weeks we were getting 150 rocks a day."
Christensen, who was luckily still in Pasadena, escaped the wrath of the guys who deliver mail to the offices at ASU. They lugged in four or five tubs of rocks a day, sometimes weighing up to 300 pounds.
By the time Christensen retuned to Tempe, he couldn't get into his office. Crates of rocks were stacked so high that "I literally couldn't get in," he said.
That was back in 2004, and the rocks are still coming in. From every continent. From 80 countries. Mostly from kids, but some for people in their 90s. This summer ASU's Rock Around the World program received its 10,000th rock, a hunk of quartz collected in the bed of the Manumati River, near Kathmandu, Nepal, by a 17-year-old student at Xavier Academy in Kathmandu.
"I probably have one of the best rock collections in the world," Christensen said.
Each rock has been catalogued, and most have been studied to determine their composition. Every person who sends in a rock gets a certificate, with the rock's number, signed by Christensen. The Rock Around the World Web site has directions for people who want to send in their prized chunk of earth.
So far, ASU's growing collection hasn't yielded any scientific breakthroughs. But that may be because no one has had time yet to really analyze enough rocks to build a small mountain. Someday, Christensen said, he expects one of his students to strike scientific paydirt.
Rocks, of course, are history books to geologists. Their composition tells the story of how they were formed, of the powerful geologic forces that turned them into what they are today.