For today's kids, it must be difficult to imagine a time before backpacks. They're ubiquitous in classrooms and on school buses nationwide, and have been for decades. But as recently as the 1960s, they weren't widely available.

Back then, the simple act of carrying stuff to and from school was difficult. "Students had no choice but to tote their textbooks and notebooks around campus with their hands," wrote backpack innovator Skip Yowell in his book, "The Hippie Guide to Climbing the Corporate Ladder & Other Mountains."

"Some tied a belt around them or clutched them to their chest as they walked," he said. "Either way, lugging study material was little more than a glorified juggling act -- without the pay."

What saved students from this avalanche of loose books and papers? The smart retooling of an existing product.

Day packs, the smaller, lighter offspring of hefty hiking backpacks, were already popular among recreational climbers. JanSport, the company Yowell began with Murray McCory (formerly Pletz) and Jan Lewis in Seattle in 1967, made its own line. It was when these day packs made their way into university bookstores that the revolution began.

By the early '70s, the sports shop inside the University of Washington bookstore started selling JanSport packs. According to Yowell's memoir, the bookstore manager suggested improvements that would enhance the product's appeal to students, and the day pack's design began to evolve.

Yowell and McCory added jam-proof nylon coil zippers and used vinyl (and later leather) to reinforce the improved packs, which sold well.

"Towards the end of the '70s," Yowell wrote, "many college bookstores were carrying our revised day packs."

Still, it took a while for the backpack revolution to cross the country. In 1980, a Harvard Law student sent L.L. Bean product manager Ned Kitchel a suggestion letter. It'd be great, the aspiring lawyer wrote, if you designed something that comfortably carried my heavy books. Two years after receiving that letter, L.L. Bean introduced the Book Pack, the Maine-based mail-order giant's first backpack made specifically for students. The design was roomier than a hiking pack and was squared off to fit books.

According to "Guaranteed to Last," L.L. Bean's official history, the Book Pack wasn't an instant hit, but "sales grew steadily as word of mouth and reviews in the media spread the story of the indestructible book bag."

The rise of the backpack signaled a societal shift. The book bags eventually spread from universities and law schools to elementary schools. And school became serious business.

Cultural anthropologist and author Grant McCracken said the backpack is a symbol of the importance of education. And the days of kids carrying small stacks of books to school in their arms are far behind us.